My T told me time and time again he wasn't going anywhere.... after about a year, he started talking about issues in his job and wanting to leave.... but always said things like "Clients can follow me if they like" so I felt ok about it.

It came out of nowhere when he said he was quitting the career itself and that all contact had to end, at least for 2 years.

So sadly, yes, it COULD happen... however try not to fixate on it, I doubt it's that common and just enjoy the time you have with them now.