As someone who has contributed to the Evennia codebase (Hacktober 22), I can say that it's OOP systems are largely well thought out and very flexible. There are hooks for things like when your object gets created/destroyed, picked up/dropped, etc.
Per another comment in this thread, it does seem like everyone wants to create a MUD but no one wants to play them. All the people in the Discord seem to have several years long projects to create "their game", but I didn't hear much about anyone actually playing any of these games.
There's definitely a bunch of us with long-term projects, but I think that impression that we don't play is mostly because devs split off into their own servers or game communities once their game is playable.
For example:
Arx is supposedly slowing down in its "old age" (it's been around longer than I've been using Evennia) but the last time I looked, there were over 50 players online.
Silent Heaven launched only a couple months ago, but it's consistently had between 15 and 30 players online every time I've checked. I haven't played it since I'm not into horror, but if it weren't for that I would definitely have tried it out. It's got its own Discord server so I figure all the chatter is over there.
Secrets of Aelandris has only had one playtesting window so far, but several of us in the Evennia community played and are looking forward excitedly to the next one. Playing it comes up every now and then in the Evennia community, but mostly we talk about it on the Aelandris discord.
Song of Avaria, which I also helped playtest, posts regular development showcases, and we've got multiple people in the Discord community looking forward to it launching alpha early next year. People ask about playing it every time a new showcase drops. (One should be going up this weekend, btw!)
The Mystavaria builders and testers are also all over on their own discord, so I have no idea where they're at in terms of activity. They aren't payable yet, but several of us are looking forward to trying that game out as well.
Machine Garden opened up a sandbox pre-alpha a while back (a year or two, i think) which got several players and active discussion on its discord, as well, although it's currently down for major code renovation.
And those are just the ones I'm personally familiar with that have names! (Aside from mine, of course, but it doesn't count yet.)
Most people who look into Evennia are looking to make a game, since that's what Evennia is for, so there's of course a bias in that direction in terms of the community discussion. But there's some games out there, and the ones that are playable definitely get players.
A lot of it is just that there's only so much free time, and those of us that really want to create MUDs wind up putting our time into that instead of playing as we find it more enjoyable and rewarding. In fact, for some it is a hobby in and of itself, ballooning into a passion project that may never see the light of day, like restoring a car you have no intention of ever driving.
People who are active in the "meta" community outside a specific game tend to want to create MUDs. It's like how there are billions of people playing board games, but if you join a board game community you are far more likely to find others who want to create them; it's self-selecting for more interest in the guts of the hobby.
The difference is that people merely somewhat interested in MUDs have dwindled over time, leaving the die-hards behind and a small, small sprinkling of new players just discovering them.
If there was a MUD that got all of the coders wanting to build their own MUD to go "this is incredible, let's just work on this instead, everyone we're moving house" -- I think people willing to do that join teams at extant games instead.
It's entertaining to create a world. It's excruciating, painstaking work to finish it and make it into something others can enjoy.
Plus, people like different aspects of MUDs. Creating it from the inside (like in LPMUDs) with others is fun in its own right, even if no player ever logs in. It's like a slow-paced, continuous hackathon, where everyone tries to make "something interesting" to show to the others.
That's been the rub with all these things since the mid-90s. They're fascinating to build and author in. But it's like a Trotskyist sect or an indie rock band or something: everyone's got one of their own, and nobody wants to join anybody else's. :-)
Per another comment in this thread, it does seem like everyone wants to create a MUD but no one wants to play them. All the people in the Discord seem to have several years long projects to create "their game", but I didn't hear much about anyone actually playing any of these games.