Because how well the slaves can follow the overseers orders is at least partially contingent on how intelligent they are, and because as NOD experiences an even more pronounced labor crunch than us, the odds of Gana filling more and more varied roles, including foot soldiers only increases.
I don't think the Nod Labor shortage has the exact same causes as ours. Nod citizens don't have the same kind of access to information that their GDI counterparts do. Throw in what's likely to be a lower supply of birth control to the general population, and a greying population isn't as much of a problem. The Regency War losses due to battlefield casualties and refugees are likely to be a much bigger problem. The worse workplace safety issues are also likely to be a serious problem.
The differing causes means that Nod has solutions that simply aren't available to us. They've cut off the refugee supply, which is a good start. Also, they can improve workplace safety conditions in order to lose less labor per turn to industrial accidents. That should have a sizable impact all on its own.
Of course, not all of their options are completely unavailable to us. They have cybernetic technology as well, although past history may make deploying it difficult for them. Ethical automation is very doable for the Brotherhood. They're already adding more automated drone units to their forces. Non-sapient Gana can also be used morall. There's a lot of jobs where you don't need more intelligence than that of a particularly intelligent monkey to do the job, especially if there's a sapient overseer present. Heck, the increased efficiency sapience brings might not even be worth the added cost of manufacturing.
Now, while the temptation to increase the use of sapient Gana in the military will likely increase, that doesn't mean the Bannerjees will give into it. There are plenty of other, more morally palatable options, as I detailed above.
Assuming the enemy will take a morally questionable course just because they're evil and that's what evil people do is dubious
at best. History is littered with times where such false assumptions ended very badly for those making them. The disastrous consequences of the shelling of the historical monastery of Monte Cassino comes to mind. If it holds up for literal Nazis, I think it should apply to the most ethical Nod Warlords we've ever laid eyes on.
And if we're taking the argument that ostensibly any heavily indoctrinated Brotherhood citizen is essentially a servitor who's lot is all about serving the Warlord and/or NOD's inner circle regardless... I suppose I'm sentimental enough to believe a creator has a duty of care- and that industrially manufacturing sapients to fill in the short fall resulting from the simple fact most humans don't want to bring a new life into such a degraded and degrading world is just to embrace the same driving forces that have despoiled the world and killed billions.
Um, I think what we have here is a failure to communicate. My point wasn't that "all Brotherhood citizens are essentially servitors and thus making Gana isn't so bad". What I was trying to say was that both Gana and ordinary Nod citizens undergo significant amounts of indoctrination. Despite that, the ordinary citizens of Nod
aren't servitors with no free will of their own, as demonstrated by the massive number of defectors we've taken in. If the ordinary citizens, who undergo more or less the same treatment, aren't essentially servitors, why should we assume that the sapient Gana are?
At that point the Warlords are just pretty much manufacturing people to use as grist for the warmachines they derive their legitimacy from, and that they are desperate to ensure don't get further outcompeted by GDI.
You make it sound like the sole reason why the Warlords are in power is because they are at war with GDI. We have ample evidence that this is not the case. The Caravanserai has been more or less neutral to us for years at this point, yet they haven't collapsed. The Bannerjee twins have sky high popular support on the Indian Subcontinent, due to stepping in when GDI outright abandoned much of the population to the mercies of Tiberium. That won't change just because peace breaks out. It sounds like Stahl also could stick around, and there very well may be others.
I'm not saying that there aren't regions where the populace is kept in line by anti-GDI propaganda that would have serious problems if peace broke out. I'm just saying that it's not the sole source of legitimacy, and there would be plenty of leaders within Nod who would do just fine without it.
As for what I want us to do? I want us to be mindful of some of where people in setting's misgivings over working more closely with some of the Warlords. Working with people out of necessity can easily become working with people out of convenience. That seems as much one of GDI's past crimes as any other given Marcion and other past GDI patsies.
We are so far away from working with Nod becoming a convenience it's not even funny. The decades worth of bad blood between both parties alone makes sure of that. Look at how much of a headache Coordinated Abatement has been politically if you want proof.
The necessity isn't going anywhere either. We need to have long-term peaceful relations with the Nod moderates. There's enough popular support for them that conquest is impractical, bordering on impossible in some cases. For example, I don't see the Indian Subcontinent becoming a Blue Zone unless Nod decides to completely abandon it for some reason.
That means we have to work with them, slowly pushing both sides towards seeing each other as people instead of the Enemy. We've made a lot of progress on that front, but it's still a long road ahead, the work of decades and perhaps generations. By the time we have achieved our goal, I wouldn't be surprised if the ethical standards of Nod have improved significantly.