Syag
I dont think anyone remained for years either.
A case series Done by a DA Shewmon “Chronic “brain death”: meta-analysis and conceptual consequences” did yield a few cases “surviving” up to 6 months. It is rare though.
The brain as you know doesnt JUST control respiration. In other words it isnt like merely replacing a non-functioning brain with a vent solves everything. Circulatory collapse is a common problem as the brain controls vasodilation/constriction for example. And even with pressors there is so long they can help.
There have ben a few cases of pregnant brain dead women giving birth. OBviously though those cases dont involve years either.