Recently I was reading Kev Quirk's Centralisation and Mastodon (via), which notes how central Mastodon.social and a few other big instances are to the overall Fediverse, making it hardly a decentralized network in practice. The article concludes with a call to action:

If you’re thinking about joining Mastodon, don’t just join the first instance you come across. Take a look at the sign up section of the Mastodon homepage. There is a list [of] alternative instances that you can join, all arranged by topic.

I think that more genuine decentralization in the Fediverse isn't a bad thing, but I also think that there are practical considerations pushing against it. To put it one way, if you're joining the Fediverse your choice of instance is a risky decision that you're mostly not interested in and are generally not well equipped to make.

Your choice of instance is risky in that if you pick badly, you'll wind up having to go through various sorts of annoyance and pain. Picking what is clearly a big and popular instance has an intuitive appeal to reduce those risks; a popular instance is probably not a bad choice. As far as actively choosing an instance goes, this is usually not what you're interested in. Most people are interested in joining the Fediverse as a whole, and one of the points of it being a decentralized network is that it isn't supposed to matter where you join. So you might as well take a low risk choice.

Finally, if you're trying to actively pick a good instance, most people have the twin problems that they don't know what they care about (or should care about) in instances, and even if they do know they have things they care about they don't know enough to how to evaluate instances. Oh, you can read an instance's policies and poke around a bit, but that may not give you clear and honest answers, and on top of that a lot of things in the Fediverse are only clear to people who are immersed in the Fediverse already. To put it one way, there are a lot of problems with instances (and problem instances) that aren't obvious and clear to outsiders.

All of this should be unsurprising, because it's all a version of the problem of forcing users to make choices in security. People mostly don't care, and even if they do care they mostly don't know enough to make good choices. This is especially the case if they're new to the Fediverse.