I don't look at a MU's success by number of players.
Also, I want to distinguish, game vs MU*. Game to me only uses code or roll of some kind for determination and conflict resolution, a MUs is an RP environment that may use rules and code. RP is success to me in a MU regardless of size. People having fun, whether all in on immersive meta or minor social interactions between their own PRPs or even sandboxed adventures.
People involved and having fun was briefly touched on in Meta vs PrP vs Planning vs Impromptu.
Its exactly what you say, enough meta with enough to spice the downtime between meta.
Though success for me is drawing in enough players who enjoy the theme and are comfortable enough with running their own PrPs. The more staff needed to arbitrate, dispute resolute, or physical have to run everything for players, the less successful it is. I want folks to be inspired enough to go out and do things they want to make changes. Meta is a mix of running a few things here and there and leaving it up to IC info distribution and letting players react.
If I run a scene or two, then post some IC info later, then folks talk and I start getting pages asking what players can do because they're interested, that's more successful then scheduling plot events time and again.
Realms had decent numbers, 60 or so unique players came by, even when I was done there were 40 players logging on weekly. I do not see this as a success. Players had fun, but there wasn't much getting along. There was meta going on, folks were interested in it (Dorset, a few minor plots on the side that I thought were even better), but the jobs numbered about 1500 in three months and every day of the 10 new ones, about 1/2 was complaints.
Coral Springs (Island), the one I'm doing now, is small, supers themed, niche. There are about 15 individual players a week, about 25 or so who have chars that check on every week or 2 at least. No complaints, everyone is out doing their own plots, meta is slow and out there, minor changes to the world (the PCs world, not the entire cosmos), and overall folks are having fun. It was barely adverstised, on a few comic places mostly, and is more word of mouth. It just seems to be more enjoyable for me to log in. This is my concept of success, I can staff and still have fun, its not a chore to log in.