Perhaps you are right. They also have server costs, but maybe they aren't as big. But other federated networks exist: Mastodon, Matrix and PeerTube. I don't think they have ads, so Lemmy should be able to at least reach their size without them. I can't say what would happen when it reaches a billion users though.

On the other hand the costs will be distributed among many instance owners, so I don't know why ads would be needed. We can have thousands of instances for example.

How do you think Blender, Firefox, Linux, etc, are distributed? Probably get more requests per day than any single Lemmy instance does.