For D&D, old as it is, this is actually a well-tread topic, and the linked data isn't the first time they've reported on it. It's been fairly well established over many editions that most groups don't play past level 10. It's why you had E6, for instance (a 3.5 D&D variant that capped at level 6, but allowed some continual non-leveling progress thereafter via feats), which conveniently hits the same popular range this article mentions.
I tend to imagine, in the D&D case, this is DM driven as much as anything, because higher level chars are increasingly difficult to balance and provide good games. On MUs these issues (for STs) are further exacerbated since you spread the PCs out over vastly different XP points. We hear about dino issues all the time, and the historical answer has mostly been to shrug and pretend it's not really a problem / "omg why do u care about stats?!?!! roleplay not rollplay hurr hurr"
Advancement is an option. You can have none at all, picking any power point and fixing people there, you can allow it but only to a point (caps), you can allow it in full, you can offer tiered characters, etc. I'm somewhat dismissing 'standard @Arkandel survey questions' but it's because there probably isn't a single best approach and will vary by game (to use the orignal example, a D&D Planescape game will almost certainly require higher power levels). However, I do think the straight D&D version of 'continually accumulate xp and grow in power until you can control the fabric of reality while some people are still hitting goblins with sticks' is very likely the worst.
If you have advancement, your ideal is probably the bell curve curve some might expect, of beginners quickly becoming intermediate (while learning the ropes, setting, game culture, etc along the way) and rarely, slowly, and with luck, some making it to advanced and then (this is important) completing their stories. This doesn't actually happen on MUs though, because of two problems at the opposite ends of the same issue: player drop-out means that a lot of newbies don't stick around, while the general aversion to character death and lack of set narrative arcs means that intermediates always survive to become advanced and then never ride off into the sunset.
So there's a variety of solutions approaches. You can not have advancement. You can have caps (we've circled back to E6). You can do tiers to have different layers of play on one game, which I actually kind of like so long as they're open to all players vs. getting an elder because you TS headstaff are a 'trusted player.' Or you can do full advancement... but not wuss out about death and other endings. A fair way back, I experimented with offering a variety of perks to players re-apping after death (including access to snowflake options), and some people did take it up.