As I say, it really depends on the ratios. It could be that for our regular troops Starblasters are the way to go. We paid character creation points for them, so if they turn out to be better value at their price point that seems fair enough.
That would potentially leave Sunblasters as a dead end for infantry in all but the very long term (when we may exhaust our pool of recruits for expanding exotics production), yes, but that's just what can happen when we're working in a realistic scenario of incomplete information. It may be that plasma ends up a technology we primarily use for artillery, or grenades, or missiles. If so, we just accept it and move on.
It's similar to how most of the infantry scale grav-weapons we've invented may never see deployment at scale.
In terms of bottlenecks, there are advantages and disadvantages to exotics. Exotics are no different to foundries or forges. They're just part of the BAP cost of setting up continuing production. What they are is more transferable than other kinds of production facilities, as what they produce can be used for more than one thing.
Let's say that you want to equip your regular forces with Starblasters.
To do this, you spend BAP to build a mixture of Starcrystal Farms and Starblaster Forges, along with Weapon, Armour and Vehicle Foundries to make the rest of their and their transports' crew's gear. This is option 1, producing X squads worth of equipment a turn for no further investment.
The alternative is that, you spend the same BAP to build Sunblaster Forges along with the other Foundries above. This is option 2, producing Y squads worth of equipment a turn for no further investment
In option 1, for a given amount of upfront BAP investment, you can equip fewer soldiers every year than in option 2. Y > X. How many fewer is yet unknown, as are what the other constraints are*. However, in option 1 you also have the option value of repurposing the Starcrystal to something else that's more urgent at the time. That option value is worth something, and is an advantage to taking option 1.
Exotic production isn't a bottleneck any more than any other production facility is a bottleneck, it's just something else we need to weigh up the costs for. In terms of production, everything, in the end, has a cost in terms of BAP investment.
In terms of the threat profile, our line troops are likely to be phenomenally outnumbered by orks. They don't just have to be able to kill regular orks, they have to kill them very quickly in great numbers, and they also need to be able to fight enemy elites and vehicles, as our regular troops are likely to be outnumbered by those elites and vehicle alone. That's what makes the Starblasters so valuable; as it gives our infantry not just the ability to one shot enemy elites, but also to sustain an enormous rate of fire so they can carve through hordes of orks and their vehicles very quickly. Our regular troops don't just need to be able to kill an ork, they need to be able to kill him, his boss, his bosses' boss, and his thousand mates. Each.
* if Steward and Warrior AP are a significant limiting factor on army expansion in the short to medium term the choice may be between a small well equipped army and a nearly equally small less well equipped army.