> Mastodon is not successful, at all.
I don't know about your definitions, but in my playbook 800,000 users is pretty damned successful. Let me reinterpret your comment for you: "I do not want to believe Mastodon is successful, since it hurts my view."
> Apparently content that was not accepted on twitter, such as lolicon, has found a new home on Mastodon.
Unsurprising. All communities end up with some version of this problem, be it hate speech, illegal content, piracy, etc. And it can definitely be harder to police in these kinds of federated environments... but it's certainly not an unsolvable problem. People just have to want to solve it. Twitter has a huge problem with hate speech on their platform, as an example, but they've basically turned a complete blind eye to it for years and years. (And even now after they've started trying to add some support for content policing, reporting hate speech still requires far too many clicks to be reasonable, given the rate people can churn it out...)
In fact, when your platform starts to have these problems, it's usually a sign you've done something right. It means your platform is actually mature enough to abuse, unlike other platforms that failed to reach this status like Diaspora. That's not a sign to give up, it's a sign that you need to start thinking about how to solve these new problems. (And they're hard problems to solve, often coming down to jurisdictional boundaries and "default deny" policies simply because the problem is seen as too hard to fix in software. You even see Google struggling to solve these problems - e.g. on YouTube it's still common that they ban content erroneously, claiming it to be extremist or whatnot.)
> The average user would never bother to migrate to decentralized services for any other reason then necessity, lack of choice, or to escape rules/laws.
Or you know, because they want to. There are plenty of reasons to leave Twitter behind: It's adding bloat that hardly any of the users care about, with yawn inducing, bandwidth destroying live video, infuriating autoplaying video, and "moments". It continues to pollute feeds with garbage from other users with no way to filter it (like "[so and so liked this]" - good for them, but this isn't Facebook...). And now they're even messing with chronology, so you get messages from hours in the past appearing at the top of your feed instead of up-to-the-minute content that you'd expect. (And my personal favorite anti-feature, the "you have location tracking disabled, we will not track your location" location-tracking sidebar filled with location-specific content.)
Looking at all of those downsides, then comparing with Mastodon which doesn't have any of those problems, supporting large messages, spoiler tags/content-under-the-fold, having decent Android clients, and decent, growing communities, and, well... it's really quite a compelling platform for me.