The comparison to Maya doesn't really work since you aren't just using a tool to make something you sell for money and thereby entitled to all, or even necessarily a large amount of money. You're using someone else's intellectual property to make your mod and sell it. If paying for Maya also came with the entire game made and IP developed for you to integrate your work into you'd have a valid comparison then. But 25% for making and selling content that builds on and relies entirely on the work of others to even be relevant? That's pretty smurfing generous. You won't get a deal that good anywhere else in the industry for that kind of work. And if people get over themselves enough for it to actually take off to some degree then congratulations to the better modders out there because Valve just made them millionaires if the payments people have received for user made content in other games is any indication.
If anyone really wants to complain about that not even two days in when we've yet to see how everything will shake out then they're crazy. And if anyone wants to call Valve evil, stupid,or whatever else for giving more content creators the opportunity to get in on the millions they've already paid out to people for things like TF2 items and CS: GO skins then I have to wonder what sort of world I'm living in.
The only legitimate complaint that I think has fallen out of this is concerns about quality control, but even then, I think Valve has wanted to move more and more to community curation for years with a large part of it being because Steam is simply getting too big to manage effectively with how relatively small they are and because they don't want to be gatekeepers deciding who can and can't put their stuff on Steam. They may not be there with games yet, but there's more than enough community rating and commentary features to root out the scammers and the shoddy mods right now making it extremely easy for those not willing to blindly spend a few bucks trying a mod to stay informed and spend their money wisely.