I don’t disagree with the sentiments behind these statements, but you’re basically asking a fish to leave their large school to join a smaller school because it will benefit the species as a whole if the species is divided into multiple schools rather than one giant school. While it might be true that the species is safer that way, it’s much more dangerous for the individual fish to be in a smaller school than to stay in the large school.
In other words, it’s safer for the fediverse for users to diversify, but is safer for the individual to join a “too large to fail” instance. If a user’s instance goes belly up and they lose their account and everything associated with it, it’s functionally similar to the entirety of Lemmy going down as far as that individual user is concerned, because unless you’re a serial lurker (in which case it doesn’t even matter what instance you’re on if you’re not contributing) who wants to invest years in a social media site just to lose your account? Most folks are going to leave rather than start all over. And most folks are going to make the decision that is in their best interests over those of some website. I know every time I see another instance get shut down or defederated, I’m relieved I decided to sign up via .world, even if it’s sluggish at times.
Switching accounts over is a big issue for sure, but even bigger is when entire communities are lost. Those are much harder to switch over, both due to scale and moderation concerns.