Twitter 2010 was already a global party.

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!

Yeah but it already failed... Popularity peaked and it's in a straight dive down.

You shouldn't be downvoted as you're factually right. MAU have been tanking since Dec 22. They lost almost 50% from the peak, and the line continues to go down, losing a few thousand active users every single day.

Which isn't a total failure, it can be a small self-sustaining network but it fails to have a broad appeal.

Twitter circa 2010 was gaining active users. Mastodon today is losing active users: https://mastodon-analytics.com/