To support decentralization and spread, should lemmy.world close registration at some point to prevent a performance overload due to too many users? Of course, if registration is disabled, there could be a hint placed somewhere near that from other instances you can interact with content on lemmy.world just like you had registered on it. There could be a link to join-lemmys instance overview.
With a larger userbase comes more people willing to donate, so it will depend on whether that revenue stream grows faster or slower than the increasing server costs. Mastodon.world has run fine on donations alone but lemmy.world will soon outgrow it so we’re entering uncharted territory for Ruud. Mastodon.social is significantly larger but I believe they also have sponsors and grants to complement the donations.
I don’t know if it still works like that, but at mastodon’s start I think the servers costs were handled by gargron’s patrons.