I have noticed a certain amount of sparser than usual activity in the last year or so. (Kingsmouth seemed to be the exception, at least for me)
I do think that the pool of mushers has consolidated/shrunk a bit, but that now that there are a handful of GMC games open people are stretched a bit thin. I've also noticed that maybe because of that, there's less people willing to start/engage in activity/PrPs, and it does seem to very much be a culture shift now where even staff relies upon active players running things to breathe life into a place. (I don't think that's horrible or wrong, BTW, just my observation).
So what happens is largely the same players are tapped into running things (or the people who would run things see this happen) on mutiple places and they burn out quick. When staff is busy and the usual suspects are not running things, it means that there's no activity. I think most people don't idle around to chat really these days because there are other things like Fallout, non-MUSH games, family stuff, ect.
I know I personally have been feeling kind of blah. I like running small group things, but when there doesn't seem to be much real chance of reciprocation so I worry about getting stuck never really getting to play my own PC, it makes me reluctant to invest a lot of time. (As opposed to TR where for all the other valid and glaring faults, there did seem to be a lot more people I knew running the type of things I liked--Nakesha and others in the Law sphere kept my energy going for other stuff because I had a soft place to fall as far as getting to enjoy being part of another person's scene they were running that wasn't a staff plot with 15 people in attendance because people were so desparate for any kind of thing to do that wasn't social).
Usually I'd be fine with investing more effort up front (typically at least for me, it does eventually come back around for me), but to be really blunt I've been plagued with some pretty serious health issues starting with complications postpartum from my oopsie baby almost two years ago that seems to have morphed into some kind of autoimmune thing I've been struggling to get answers for (looks like maybe there is some hope soon, keep your fingers crossed), I've picked up a second job, and I'm trying to balance once more being a parent to a young child and the increase in household stuff that needs to be done alongside that after enjoying a handful of years of just parenting preteens/teens. It may be since the MUSHing crew seems to be aging a bit (when I first started mushing there were a ton of teens/early 20 somethings--many of us are still around and...not anymore) I bet that there are quite a few people in the same boat of needing to balance more life stuff. (From friend chatter, especially needing to pick up a second job, it's the decades where more health stuff happens, and then there's family stuff like children/divorce/ect.)
I have not noticed a huge amount of actual activity decrease--just less public chatter because people are doing things other than hanging out ooc chatting on windows. But every year that I've mushed there is a dip in November/early december (used to be family centric breaks or huge work increases/finals) with a bump up a little during the holidays when people were home and off work/school, with another bump mid January when people were bored and it was crappy outside. I think that's still true to a certain extent.