You can not buy it all you want, but it's true. Consider that even if you were right that it takes only a few hundred users to require shoveling TB's of data (it's not, assuming we're talking monthly, unless you happen to be hosting e.g. George Takei and similar), the ~$5/month VPS I run my instance on can do that just fine at no extra cost. It's not necessary to pay insane AWS egress rates.

> For every involved server.

This is misleading. Not every server sees anywhere close to every post, because most servers are not "involved" with any given user, and that becomes increasingly true the larger the network becomes. Larger servers sees a large proportion but also receives only one copy of each post even though they're likely to have multiple recipients.

Realistically, the number we're looking at for the source server is about half the total network cost for that set of users.

> This just doesn't scale, no matter how cheap bandwidth and storage will get in the next years.

The cost per user of running a Fediverse instance is dropping, not rising, even as the network size has exploded. And as it is, the main cost of running e.g. Mastodon is that Mastodon, fittingly, is a huge, inefficient, resource-hungry beast. Not that federation is inherently costly.

My prediction is that the average cost per user for bandwidth, compute, and storage will be <$0.10/month within a couple of years. It's probably below that already if done efficiently enough at reasonable scale.

The bulk of the cost will be admin, moderation and ops, which scales linearly or sub-linearly with number of users.