I don't think that you are replaceable, I don't think he will forget you, and I don't think that the feelings that he expresses to you are "fake."

The reason I think this is because of an experience I had right out of college. It was different in most ways than being a therapist, but the same in others. I took care of people with developmental disabilities. I did job coaching with them too. I was paid to do it. It was a very one way relationship. They needed me in a way that I didn't need them. But I loved them. For real. Over 20 years later I still think about them pretty frequently. I wonder where they are and if they are OK. When one of them died unexpectedly I grieved for him. I still think about him too. He was really fun. Exasperating sometimes, but still pretty fun.

I'm sure that if a therapist sees a client for two or three sessions, that client probably blends into a lot of the rest he only sees a few times. But I imagine the ones that he sees frequently for months and years, he remembers and really cares about. How could he not? It isn't the same as a "real" friendship, but I'm not sure that it is less than a friendship either.

My guess is that your therapist told you that friendship wasn't possible after therapy, not because he doesn't really care about you, or because he doesn't think you are someone he could be friends with, or even because he didn't want to get into trouble with his licensing board. My guess is that he told you that because he felt like saying "yes" would be harmful to you. Maybe he feared that you would "rush" through the therapy phase of your relationship to get to the friend part. Or maybe he imagined a scenario where you and he were out with other friends and he accidentally mentioned something that he only knew about because of the things that you shared with him in therapy. Anyway, it's a possibility.