The keys are not unlimited. They cracked down on this quite a while ago. You have to submit a request for keys which Valve may or may not approve, and they will definitely not approve a request for 25K keys on a game you have sold 5K copies of.
Most of the reporting suggesting this was coming from spam games that sold literally <10 copies on Steam and requested 100k copies for Russian penny bundles. If that foreshadows further cracking down, then fair enough, but as-is it's fair to say that any legitimate commercial request by a developer would be approved.
