On the other hand, flip that around.
If we don't do something like Karachi, we are leaving powerful Nod warlords unmolested to reinforce whichever of their fellows are most able to take the resources thus handed to them and shove them somewhere uncomfortable for us.
If we actually hurt someone like Gideon enough that it's realistic for us to finish him off with a continuous steady push, it may not be the best use of our resources to throw everything we've got into crushing him and risk getting stabbed in the back from elsewhere. GDI may be better at the "strike across intercontinental distances and have one Blue Zone help out others" game, but that doesn't mean Nod can't play it at all.
This applies less to places like Western Europe or Australia, where there just isn't enough actual Nod force present to be a major inconvenience to us... but by the same token, we gain less from decisively and semi-permanently defeating them there.
The big problem then becomes balancing our desire to continue offensives against warlords who are losing, versus our ability to mount a defense against warlords who are not losing, versus out ability to continue to sustain that defense against warlords who aren't even meaningfully engaged with us and can wield their forces freely to bolster whoever they please.
This is kind of an inevitable consequence of Steel Vanguard. As long as the Great Dogpile was expected to involve GDI doing the usual sort of turtling while various Nod warlords competed to see who could come up with the most dramatic way to kick us in the shorts, the warlords had reasons to operate separately with minimal coordination. But now that they are all directly or indirectly threatened, we can expect to see (most of) them coordinating more closely in self-defense.
The thing is, there's two big separate issues:
1) Is Reynaldo, personally out of the picture?
2) Are Reynaldo's forces, the actual Nod guerillas in Western Europe, out of the picture?
Reynaldo is a cunning tactician, but he's not Nod's only one such. He'll be useful to Kane, but not indispensable; other Nod generals could do about as well for him. His personal head run up on a pole is not that important an objective to us, even if in the future we wind up cursing his name as Kane's left-hand man... because someone was going to get that job even if we blew Reynaldo up.
Beating Reynaldo's forces, even in his own absence, has real strategic impact.
If we are waging war in Europe and North America, we are fighting in our "home". It will be effectively impossible to interrupt our supply lines, while any Nod reinforcements coming from outside the theater will have to run the risk of interception. Nod can't "wield their forces freely to bolster whoever they please". Even if we set aside the major difficulties of coordination between warlords with different doctrines, which isn't a small thing, GDI has control of the sea and the air in America and Western Europe. There's no guarantee that Nod forces can get in, and if they do arrive safely they will be fighting in GDI's backyard.
If we are waging war around Karachi, we are fighting in
Nod's backyard. Krukov and Bintang can reinforce Nod India immediately, while our supply lines could be politely described as "tenuous". We will be fighting very far from home, and our shells and food and medicine will have to run the gauntlet between Mehretu and Bintang. Nod India will be fighting on home territory, with all the shells and food and medicine they need from the factories of India, and Nod's warlords will do everything they can to keep those factories running.
Now, there is a real risk that while we are distracted in America and Europe, some enterprising Nod warlord will launch an offensive somewhere else, and we'll be in trouble. We can absolutely get stabbed in the back. But that same risk applies to India, where we will have to establish and defend a Planned City right next to multiple major Nod warlords. No matter where we launch an offensive, we'll have to worry about the warlords who aren't being attacked deciding that now is an excellent time to attack
us. So why are we planning an offensive against an unknown enemy very far away rather than focusing on a known enemy who is very close?
You're right, our chances of progress on a hypothetical Indian or Iranian front (remember good old Mr. Nuke happy?) are low and I wouldn't want to press on those fronts with more than raids and scouting expeditions until we wrapped up other fronts like the Americas, Australia, and Europe.
But we also don't just want to leave the 'Shah of Atom' or the mad scientists making Ghana in India alone. Letting them build up unopposed is not viable especially as Ibrahim has ties to the Caravanserai according to 'The Regency War: Part 2'. My goal for Karachi is to have it be a thorn in their side, preventing them from supporting the other Warlords to the extent they are currently.
By restricting their operations, being a base for raids against NOD in the area, a central hub for combating Tiberium in Southern Asia, the economic benefits of more closely tying BZ-18 to our economy, and acting as a beacon to those dissatisfied with NOD depriving Ibriahim and the Indian warlord of their manpower (and saving lives from Tiberium while we are at it). The goal with Karachi isn't to make substantial progress against NOD in the region, it is to do the same thing to them that Chicago did to Gideon, distract them. Get them to spend their energy on focusing on Karachi instead of helping other Warlords like they have been doing.
I know that wasn't the origional intent behind Chicago, but good lord that has been one of the major side benefits. How many times has he attacked the Windy City again?
As far as I can tell, the "Shah of Atom" is a second-string warlord whose substantial atomic arsenal is his only real advantage. He isn't notable for going on the offensive against GDI, and his friends in the Caravanserai are currently involved in a shooting war with another Nod faction. I'm perfectly happy to leave him alone.
India doesn't have to interrupt their support to the other warlords because we build a Planned City at Karachi. They can keep sending them Gana, and in exchange they can ask Bintang and Mehretu to attack our long and vulnerable supply lines.
Karachi doesn't restrict India's operations. Raiding Nod would certainly be valuable, but a Planned City is expensive, and India would be extremely capable of hitting any harvesters we send out into the area. The more I think about it, the more I ask what the actual benefits of Karachi
are. Unless India decides to imitate Gideon and suicide attack into a fortified position, having one Planned City in the area doesn't accomplish much.
Karachi would let us take in Indian refugees, but we don't exactly have a shortage of refugees. America and Europe and Russia and South America...we've probably absorbed millions of people already by driving through their towns.
This one of my major reasons to want to see it done. After seeing how how much Ganas affect the war, doing something to disrupt their deployment would be more useful than simply letting them come in directions they choose.
Because like most defensive stratagems covering vast areas, it's much easier to concentrate forces to attack where the enemy is than to protect every important location on one's own side. That means even at seemingly greater shorter-term costs to perform the attack, I'd say going for Karachi could save many more lives overall in the long-term than letting the enemy continue dictating where they choose to attack from. Force them to defend instead for once.
Otherwise the war will likely take longer to resolve, and every turn the dices are rolled is one where Nod could grab the advantage instead by simply having better luck on the rolls.
In that sense investing in more mobile assets such as ships and drones will likely make bigger difference than anything else, along with Zone Armor so the frontlines can afford to handle urban defense and invasions with more ease by having armored infantry that can work closer with artillery and air strikes to provide a greater potential concentration of force against small but tough enemies such as Ganas, Walkers and maybe even the psi-commandos if they attack again.
India doesn't have to attack Karachi.
Securing a Planned City at Karachi wouldn't immediately give us the ability to conduct an offensive into India. Karachi does not force them onto the defensive. Unless we're planning to use Karachi to mount a major attack on India, which would be a serious gamble, all Karachi gives us is...better intelligence on India?