@BobC7000 The micro-blogging part of the fediverse never was around having an endless conservation of toots/notes/whatever_the_term_your_system_uses.
Masto and microblogging on the fediverse on a more global scale is around the human interactions, not being a journal of everything that was wrote. Services like blogs are more prone to that. Don't forget that for a service like this one it's also lots of datas that are hard to host, as they cost a lot of money to the admins. That's why some instances have policies that allow to automatically delete posts older than X days.
Migration by itself on Mastodon specifically is quite fine (just create another account somewhere else and switch something on your parameters).