Keep in mind that AIs are a new race. As in, they just came into being in living memory. One of the first places that they would be pressed into service is in hazardous environments for humans, including combat. This would be especially true pre AI civil rights. But that doesn't mean the entire military structure is going to be dismantled in favor of a new, untested and unknown technology. Especially in the military which can be resistant to change when they have something that works. I also doubt that the introduction of AI combat units would mean they would immediately or even quickly retire their active duty soldiers. Given the relative youth of AIs as a species, then the most likely case scenario is that they would be assigned to existing squads of humans, as a training and monitoring tool, to see how they perform. And given the relative durability of an AI combat chassis, they are the one most likely to survive if something catastrophic happened to their squad. I imagine cases like Bubbles' are not at all uncommon. PTSD and survivor's guilt are very real and serious things. It can be hard to open up to people again. Especially people who don't and can't understand what it's like.