I once asked T if he was going to stick with me through this. His answer was "I'm in." He has since said, repeatedly, that he has no plans to go anywhere and won't ever terminate me against my wishes. He's also promised that I cannot push him away, no matter what I do (short of physically harming him).

I like how he put that, because, as you say, he really can't promise that he always be there. Too many things could happen that he can't control. All he can promise is to behave a certain way.