This is true, but it doesn’t account for one of the major use cases of social media: connecting with specific people and groups you know IRL.
People are not fungible, and thus social networks are not fungible either. Social media lives and dies by the network effect.
This is less an issue for Lemmy as it is for Mastodon, I guess. But even so, Lemmy has yet to reach the point where you can find an active community on almost any niche subject, like you could on Reddit or Twitter. Hopefully we’ll reach that point eventually, and it would be a crying shame if it was then torn away by one dominant instance deciding to close up.