How will they destroy it if they are literally only making it larger and more useable. The average normie today has no reason to create a mastadon account, because almost no one uses it. But if they can use a major social media platform, and use a federated free service like mastadon, then that makes mastadon much more valuable to them.

The fact that a social media service is objectively good doesn't mean anyone is going to use it. Having backing from the biggest social media company in the world, might actually get people to use it.