Twitter is like CompuServe and AOL, Mastodon is like the internet. One based on proprietary systems, the other based on open standards like ActivityPub.
History showed that centralization is ultimately extremely fragile.
but how many people today use Discord et al. compared to IRC? I think the centralized-decentralized thing is a cycle, not a one-way logical progression.
Which is why I believe that once the fediverse has fractured along ideological lines, as it inevitably must, the end result won't look much different from the old internet: big influencers running their own site/blog, with loose connections to ideological allies and a bunch of commenters in orbit.
And that's a good thing. Individuals get to choose servers that align with their views on moderation and morality, server moderators only need to deal with occasional maliciousness rather than trying to walk a fine line of tolerance, and your social group isn't at risk of being disrupted by a billionaire moving in and blowing it up.