As I see it, Messenger Plus! is as good as feature-complete. However, you yourself said that in the recent beta you had made a technical step that would allow a whole new array of functionalities to be added in a future.
I would personally feel sorry if MP! was discontinued. However, I'd be proud to have helped as much as I could to make it better at some point of my life.
But: Messenger will continue evolving. Apart from all the compatibility issues that will be fixed, there may (and will) be new features to be potentially enhanced by Plus!.
But this shouldn't force you to stay. Actually, I think the community, the vast community this application has achieved, could take control of the development at some point, and while a certain reduced group of selected programmers do the hard work, the rest could be suggesting, critisizing and by all means taking the decisions on how to move on with Plus! (well, maybe not that far community-based, but I believe a group of selected users with proved common sense could judge what has to be done next). This is somewhat near to public domain, but doesn't actually free the code itself. Of course things can go bad, but we wouldn't know if we didn't try.
This is just my somewhat late 2 cents to the thread.