I'm really not worried about Mastodon scaling issues at all.
When I left Twitter in 2008, we had roughly twice as many users as the current combined Mastodon network, all running on one MySQL server that had the same specs as a high-end 2013 MacBook Pro, plus roughly 10 web servers and 5 queue servers.
To be fair, growth wasn't as rapid, and we had local-infra advantages over federated systems, but these problems are solvable and I have no doubt will be fixed soon.
*Hugops to all admins!*