Diaspora* is proof positive of that. I log in like once every 3 years at this point and it's all still there.
I'm interested to hear your perception that fedi is shrinking, I don't share it (despite knowing that "Mastodon" MAUs - however they're tracked - are down from peak) but I also think I have a sort of bottom-up approach to finding ppl to follow after being here almost 10 years and also being distrustful of Firstname Lastname clearly-has-a-mortgage accounts who post Takes.