we kind of skip the most expensive costs

I'll be honest, the reason I skipped over this is because it's entirely unsubstantiated. Show me an up to date breakdown of reddit's costs that shows how much is exec pay versus infrastructure costs or whatever.

We're talking an entirely different scale here as well. Anyone can host Lemmy on an old computer and that'll work to a point, but as more users join you'll need a bigger, faster computer. Eventually you can't make a computer any bigger and you need to start offloading key components on to separate computers - one for your database, one for your image cache, etc.

Before you know it, you suddenly can't make the database computer any bigger, you need to split that off into more and more complex architecture. You need more caching, a CDN to make sure requests don't hose your CPU and the site remains fast. All big Lemmy instances will hit this issue with scale sooner or later. When that happens, it starts to become a full-time job just looking after that infrastructure, which is now going to cost you thousands of $ a month. Then you need to factor in that the lemmy software isn't currently built for that kind of scale (It does scale to a point, don't get me wrong), it's built to scale out in instances, not to scale single instances. It wouldn't take a huge amount to add in the code necessary to help an individual instance scale out, but someone has to write and maintain that code, which in itself starts to become a full time job or several full time jobs.

The future of lemmy is bright, but it will hinge on users supporting their communities and if they don't, expect things like ads to appear.