The best comparison here is email. So Google created an email service and serve ads with their email. This, of course, has no effect on anyone not using Google's email service and since Google still follows the standards protocols for mail servers to talk to each other it doesn't even impact their ability to communicate with other remail servers or vice versa.

The same idea works with Mastodon and Lemmy - there isn't one centralized service that can be bought, the same way you can't buy "email" or "the web".

The absolute worst case would be Musk buying the rights to the server code and changing the license for all future versions to something closed. At which point, everyone else just forks the last open version and builds on it from there. So long as neither changes the underlying protocol this has no real effect on end users. If one of them does, then it creates a split between Muskodon and whatever the new fork is called akin to both sides defederating each other.