It's absolutely beyond me how people believed (or still believe) that Mastodon will be a replacement for Twitter. That's a level of delusion I will never get behind.
I mainly used Twitter to see what was going on in tech & open-source and discuss development topics, and comfortably 50% of the tech people I followed on Twitter have now set up on Mastodon, either additionally (cross-posting to both) or in quite a few cases as a full replacement. I post the same thing to both and I get far more responses & engagement on Mastodon now. For me personally it _has_ replaced Twitter.
Depends which circles you move in and what you use Twitter for (it's certainly not useful in the politics & journalism scene yet, for starters) but there's already significant movement here in some areas.
This is the single greatest barrier to entry that's not going to go away without compromising the ideals of federation.
If you want the masses you need a "classic" setup: big commercial company, lots of money and a centralized infrastructure.
Why do people only consider Mastodon (or many Open Source Software) a success when Joe Average wants to use it?
There is also the tendency for the mainstream successes to suck all the life out of niches. It’s not inevitable, and becoming a mainstream success can itself remove what was valuable to you in the niche.
But if I want to use Mastodon, I’d rather all the people I want to follow are on there.