If you care about your account and your post history, you're free to run you own instance. People are also working on mechanisms to sync community subscriptions between accounts, which would at least help the UX navigating multiple accounts. Lemmy also has an API that I'm sure users will be looking to create services to backup your content as well, if something like that doesn't already exist.

As for defederation, I'm only aware of these major reasons and all of them are legitimate:

• Nazis
• NSFL/CP
• Sourcing illegal content (illegal from the defederating hosts side, and this really lends to the above two as well)
• Growing pains as Lemmy explodes with new users

The only thing a giant corporation could do is muddy the waters, but people who care about the future of Lemmy are working to prevent that https://wedistribute.org/2023/06/fedipact-blocking-meta/