"Works on my machine" isn't going to cut it for running a popular social network
Nobodies going to go around searching for mirrors, they'll just leave and go back to twitter
To understand how it's decentralized, search for @jonty@chaos.social on your own fediverse home server and the post will pop up.
There's a particular user story here which is a bunch of people who don't have accounts on any server, wanting to see the content from a central location (chaos.social). I do think it's worth talking about this story and ways to fix it but it's not really accurate to blame it on the federation behavior.
You can't post (because you're not logged in), so there's no issues with moderation. The toot is already federated publically, so there's no issues with unintentional read access. It doesn't need to contact the original server, so there shouldn't be any load/DDoS issues. I must be missing something…