The title has much more significance than we think.

Obviously Luke is the last Jedi, what I believe the title means is that he will ALWAYS be the last Jedi. As in, that's where the Jedi line ends. Rey will end up being a new type of light side Force user.

And no, it's. Its not unrealistic. We already have the Knights of Ren which are new types of dark side Force users other than the Sith or the Inquisitors.