I think therapists and clients can certainly have successful relationships post termination, but those probably don't get discussed much. Things usually become public when someone makes a report, and that only happens when the relationship ends badly. There is probably a high risk of these relationships ending badly, which is why the profession has chosen to impose limits on what professionals can and can't engage in.

It's true that, in general, there is a power differential in the therapy relationship. But power differentials exist in any relationship where there's a difference in feelings. Whoever likes/loves/needs the other person more has less power. This happens in romantic relationships as well as friendships. But between two people, the power dynamics can shift over time. For me, I tend to fall in love quickly and crazily, whereas the men I fell in love with tended to take longer to develop deep feelings for me. Usually by the time they fell in love with me I was falling out of love with them. So in the beginning of a relationship, they tended to have all the power, whereas in the end I would have all the power. Power differentials are quite common in various relationships, and I don't think it's necessarily fatal to the relationship although I don't think it's ideal either.

I've never wanted to be friends with my therapist. I actually can't imagine being friends with him, partly because he's a lot older than me and we always had a parent-teenager dynamic and partly because we didn't really have much in common other than the fact that we're both well educated. He was cultured. He liked art and music. I tend to be somewhat low class in my interests, preferring drunken dance parties, cards against humanity, horror movies, and electronica to art shows and classical music. So I really just can't even imagine a friendship.

But I think people who are closer in age and had common interests probably could be friends. There is a high risk of things going wrong, but the risk isn't 100%. I think it could work in some cases. Waiting for a period of time is probably a good idea though.