Twitter has two killer features: the people who use it, and search functionality that allows you to get near-instant results. Mastodon (and I assume Bluesky) cannot compete on either.
Privately accessible content tends to be much smaller so local caching works. And if communication is not meant to be instant, email proves that a decentralized system works.
What gets a little more difficult is content that is widely available but behind login. Facebook has this but twitter and mastodon tend to not be this way.
In fact mastodon is a workable system for decentralized social networking, so I think the skepticism is unwarranted.
The only question in my mind is whether a popular social network can be effectively bootstrapped without a core capital incentive.
Or the difference between 95% of the results of a search and 100%, especially if the 95% is missing the least popular 5%.