(although given the decentralised framework of the fedi, I’m not sure how that could even happen in the traditional sense).

It's possible to dominate and softly-control a decentralized network, because it can centralize. So long as the average user doesn't really care about those ideals (perhaps they're only here for certain content, or to avoid a certain drawback of another platform) then they may not bother to decentralize. So long as a very popular instance doesn't do anything so bad that regular users on their instance will leave at once and lose critical mass, they can gradually enshittify and enforce conditions on instances connecting to them, or even just defederate altogether and become a central platform.

For a relevant but obviously different case study: before the reddit API exodus, there was a troll who would post shock images every day to try and attack lemmy.ml. Whenever an account was banned, they would simply register a new one on an instance which didn't require accounts to be approved, and continue trolling with barely any effort. Because of this, lemmy.ml began to defederate with any instance which didn't have a registration approval system, telling them they would be re-added once a signup test was enabled.

lemmy.ml was one of the core instances, only rivaled in size by lemmygrad.ml and wolfballs (wolfballs was defederated by most other instance, and lemmygrad.ml by many other big instances), so if an instance wasn't able to federate with lemmy.ml, at the time, it would miss out on most of the activity. So, lemmy.ml effectively pressured a policy change on other instances, albeit an overall beneficial change to make trolling harder, and in their own self-defence. One could imagine how a malevolent large instance could do something similar, if they grew to dominate the network. And this is the kind of EEE fears many here have over Threads and other attempts at moving large (anti-)social networks into the Fediverse.