I don’t think they will close themselves off. I think we will see three ‘levels’ of instance. The big core instances (a handful) which have dedicated teams running everything (might be volunteer, might be staff), a fairly large smattering of small instances ran by corps (the fedverse is a social media platform after all), as well smaller groups of like-minded people (eg beehaw or lemmygrad), and lastly the hobbyist who want to self host.