I agree with you: I think decline of a site is an inevitability, especially after advertising is needed due to increased traffic.
But I personally don’t need Lemmy or anywhere else to be permanent, since what I get out of it is either transient (scrolling for memes and things that pique my interest) or meaningful enough that it remains with me, meaning enjoyable or thought provoking discussions.
Granted, I’d rather alternative sites not go tits up in rapid succession while the shuffling corpse they’re trying to ape continues to slog on mindlessly, but keeping the impermanence in mind makes it easier to see these places as areas to congregate rather than the end to surfing the web in general.