It’s actually pretty easy to argue it’s fair once you look at everything. Steam offers a shit ton of resources for that 30%, including hosting, distribution, patching, workshop, etc. And that’s not even getting into the fact that the dev can get all of that AND get steam keys that they can distribute themselves (meaning valve doesn’t get a cut of that) that still utilizes the same infra.
I wish I could find it, but I recently saw a video of Thor (@piratesoftware, does his own game dev and used to work for Blizzard) talking about this and going into even more detail than I can remember at the moment.