I mean that's already true to a degree. Pulling the plug on someone in a coma is a fairly normal thing. Unlike what you imagine a coma isn't a binary thing, there's degrees. When does it become okay to pull the plug?
Intentionally breeding people that have no intelligence is a very different thing and I don't even know why we're talking about it as if it's even remotely similar.
> The reason why you'd pull the plug is because there is no prospect of survival
Same goes for a body bred to have no brain, surely. Of course that moves the dilemma to "is it ethical to create humans with no possibility of survival", which is a different question equally worth debating.