Since the admin salaries aren't counted in the .world figure, it doesn't matter how well that scales. That's why we are trying to take them out of the equation.

Regardless of the actual numbers for reddit (which we plainly just don't have), we do have numbers for plain old forums (e.g. phpBB) which provide a very similar service as Lemmy. And these numbers are far lower that what the .worlds pay.

The main issue here is replication. Each instance needs to store everything of all instances. That requires a ton of storage and that's not for free. With a network of conventional forums you don't have that issue.

Or to put it differently: If Lemmy was the size of Reddit (1.2 billion monthly users or roughly 32000x the size of Lemmy right now) and the number of instances scaled accordingly, there would be about 11.6 million instances, each hosting a copy of almost all the content. That's a crazy amount of replication that increases the cost for everyone enormously. Because not only are there more instances that need to pay hosting costs, but the costs for each instance balloon as well.

But there's an even bigger issue with the replication: Since everything is replicated, the owner of an instance can get into legal trouble for illegal stuff hosted on the instance, say e.g. illegal kinds of pornography or e.g. in the UK any kind of age-restricted content if they don't do age verification.

That means an admin can't just rely on other instances doing their modding correctly, but effectively every instance needs to moderate all federated content too.

That is a strain for 40k users (enough strain to e.g. close down lemmy.ee), but it becomes entirely unmanageable for 1.2 billion users.