There just needs to be automatic instance assignment for new users with registration time opt-out for those who want to choose. Pick whichever one is the most populated. If that’s down at that particular moment in time, pick the next most populated and so on. Take the instance picking out of the equation for people who don’t care about that. People who do care can still pick their own.
That being said, it’s difficult to grow a new website in general unless it’s already got established backing from other source (i.e. a large corporation, celebrity, etc.), It’s not like old days in the 90s when there were far fewer websites in general.
I’m sure something else will eventually come along. The only way to stop emulators from existing is to not have a system at all. I.e., if Nintendo were to stop making video game hardware, then there would be no new emulators except for ones developed for previous/existing hardware.