Their absence can break the immersion has well, take Baldur Gates 3 with everybody talking with a voice over except the main characther, with generative AI now they could have went around the challenge to have to record so many possible voice because of the custom characther creation.
Text-to voice is a different form of course than generating what is being said... but it could also tweak a main script to customize to the specific in-game situation, to reflect better what is going on, often cut-scene dialogue will not match the current injury level of everyone or does make sense in the context of what just actually happened, it could be one of its strenght, worse dialogue but way more immersed into the situation, because it took everything that just happened and currently going into account.
The dream sounds awesome. But, as with everything, the devil is in the details. If the characters start acting way out of context or with different personalities to the script then it will ruin the game for a lot of people. But, you're right, that at times some NPC's seem oblivious to what is going on. So, I think in this case we'd be trading one good thing for another good thing and might not get a gain overall. I think whatever they do has to be done slowly so it doesn't ruin the game in the process. But, given the current climate I expect some major screwups before they sort it out. The whole "go big and break things" is the modern technical mantra.