To be a little more verbose, I’ve just found that banking on shared backstory with others sight-unseen (particularly people you don’t know that well) is just an unreliable investment at best for all the reasons that the OP outlined, plus the “shit happens” effect where you just have to conceptually step around the idea that people will idle out or whatever.
Also, most of the best RP and character chemistry I’ve had I’ve just worked out on the fly after a couple scenes I’ve had with someone else on the grid. I’ll happily work on backstory with people then if I feel it fits but the above requirement is something that I would just not mess with. Similarly if I run into a MU* where every character already seems to know each other for ages and I feel like I just stumbled into a group of pre-existing friends at a party I’m strongly compelled to not crash it.
And as a side note, if gamerunners are that worried about “bad actor” problems (which is very much a thing), 1) How about making it a roster game (this is very much not for me but I know some people find this helpful), or 2) Why make a fully public game at all?
Again, if this works for people, good going and I’m glad you’re having a good time. This just isn’t for me.