Then X will become the only social media as Musk can keep it free unlike any competition and use it to push politics he likes or finds it beneficial for his other companies. In fact, according to reports X is already not making much ad money so it’s already there.
There's already free ad-free social media, see countless services in the fediverse
Who pays for the costs of those and why?
There are many Mastodon servers run by ordinary people simply because they want to. And before the shit-show the internet has become, there were many forums and IRC channels, absolutely free, and with 0 ads.