I think that enshittification on any SM platform, whether free and open, or built for commerce, happens when companies try to exploit it for commercial gain.
Take Usenet for example: At the beginning it was great, then spammers found they could post unlimited spam across the newsgroups for free, and it became shit, barring a few groups where mods had to work very hard to weed out the spam to keep them readable, but eventually collapsed, and people moved on to the new platforms.
Reddit, was built for ads and tracking its users to start with, so the gradual creep of enshittification was no surprise there.
And now we have nation-state backed disinformation campaigns to deal with in addition to commercial spam.
I could see Lemmy and the Fediverse in general taking a similar path to Usenet, if the devs, admins, and mods aren’t vigilant about keeping bad actors out.
I like the Fediverse’s guarantor feature for adding new instances, but we’ll have to see how well it holds up under assault from spammers.