Unlike others, I'd expect a signicifant decline of posts as not only many users but also loads of communities would be lost. That's why from my perspective users and communities should be evenly distributed across instances.
On top of that, there should be a feature to move entire accounts or communities to other instances. That way a community including all its members could just be migrated before a major shutdown.
Similarly, I think it would be a huge disturbance for the email system and possibly the entire internet world-wide if Gmail went down next month even though there are in theory plenty of alternative providers. Or supermarkets. If the IT of Walmart, Visa/MasterCard, Amazon AWS, Microsoft etc. have an outage it always has huge impact.
Lemmy as a whole isn't that big and far from being critical infrastructure but we all want it to grow we should bear in mind that huge central services are always more risky than many small services.