And so is Mastodon, now.

No it isn't. I know someone who runs a specialist news outlet and cross-posts everything to Mastodon. It yields only a trickle of traffic/engagement despite the target market being pretty anti-Musk. Indeed, I'm told traffic/engagement on Twitter has actually improved since the takeover. What people say and how they behave are two different things. A lot of people still check Twitter every day because train wrecks are interesting.

Every time someone shares a link to a Mastodon post, I click on it and eventually realize it's not my instance, so I can't just click to follow the person or return to my feed by clicking. And that is only one example of how the decentralized experience is fundamentally broken.

There also weren't any functional alternatives to Twitter at the time. The following+unthreaded discussion model has only been replicated by Mastodon, and Mastodon obviously has much stronger competition!