Well that would lump all user into one instance. Costs of that instance would be huge, leaving the owner to seek out ways of monetizing the whole thing. Downtime and disruptions would then also impact a large part of the user base. And if the instance goes belly up, those users are shit out of luck. It would also place a lot of power in the hands of that instance. Basically what you describe are the exact things the fediverse is meant to fix. Otherwise you are just creating a new single large social media, with all the same issues. The whole point is to have people be distributed over lots of instances or even run their own instance. The point however is valid, this is an issue that deserves some thought. But it’s a hard problem to solve. This is true for more aspects of Lemmy in my opinion. For lots of things it’s too similar to other larger social media, where I feel better choices exist. Or it’s similar to one kind of social media, where it could use aspects of other kinds of social media. We are seeing some of that being fixed now with Lemmy being able to integrate with other parts of the threadiverse.