I honestly think that MU* s could offer something new to RPers who are looking for something lightweight, especially on tablets and phones. It doesn't need constant attention, it doesn't need a specific console or computer, and/or your internet access is limited or spotty, MU* s are great. Telnet seems to connect okay, but I've never tried to stretch it beyond 'connecting in to chat while cooking'.
When it comes to systems, I come from the other side of the MU* coin, where combat systems and dice rolls really weren't a thing, really. It's only been in the past decade or so that my circle's kind of accepted the benefits of combat systems and the like, but every system we add onto that is largely based around 'if the system's results disagree with the fact that you got hit in the head by a hammer, RP the fact that you got hit by a hammer'.
The new players I do see tend to come in due to a setting incorporating a theme or piece of media they like. The more complicated and rigid the system, the more overwhelmed a person may be, but that tends to be a thing even for MU* rpers in my neck of the woods. One thing Dream Chasers MU* did was have a test area that guests could access that had Digs (a system for helping GM dungeon situations).
As for overpopulation, I think applications tend to help in MU*s I've been on, as long as new players understand it's less 'job interview' and more 'will your character be able to fit the theme and thrive on the grid'. If someone apps a loner who drinks their darke ale in the corner of the tavern without more hooks or attributes beyond that, staff's going to offer suggestions and send the app back. If there's an excess of a specific trait or type in characters that isn't meshing well with the theme, they may be added to the 'restricted' list.
I used to think MUs were on their way out, but seeing more and more preferences for different and unique types of gaming that are free, not tied to a specific site that could change & make RPing impossible (ie Tumblr) and flexible for players (not talking code-wise, as that's another bag of worms), I'm beginning to wonder if there there's a lot of attractive qualities MUs can have, as long as we're aware of them.