There are fewer computer users, when you look at it by proportion of the population, since most people who aren't into PC gaming, programming, video editing and similar have switched to just using phones and tablets.

That said, there are still plenty enough people to keep the fediverse going, and frankly I don't think it needs to be nearly as unintuitive to the average user as it is. That's a design problem.

Granted, I've thought the same of Linux for ages. It could be as intuitive and user friendly as windows... Except the people who create it are largely nerds who cater to themselves and fellow nerds, and who even take pride in using a relatively inaccessible system, which results in both the absence of basic features (like no color blind mode! In 2023! C'mon) and forums that are mean and condescending to anyone who asks a question (not everyone is like this, of course - there are people who genuinely try to help others get into Linux - but there are enough other people doing the opposite that it's very unpleasant to deal with as a newbie.)

All of which is to say, whether the fediverse can become mainstream or not depends on whether it can overcome its own nerd culture and prioritize ease of use. I hope it will, but Linux hasn't yet, even after all these years (although it is a little better, arguably, at least). We'll see I guess.