At that point, Valve can do whatever the hell they want, and everyone has to accept it. When Valve declares that the 75% take is rising to 80%, 85%, or more, modders have no recourse. "Well, you can go back to making mods for free then! Bye!" Valve can dictate prices, market strategy and viability to the modders wholesale.
What's more, they can do the same to game publishers. "Oh, Bethesda? Yeah, you're only getting a 10% cut now instead of the 20% you had been getting. Oh, you don't like that? That's fine, we'll just stop allowing your games to use our service." So Bethesda has to take the cut. Or has to pay extra to get on Steam, or however else they want to do it. And since Steam has such market dominance on the PC console, Bethesda will fall in line and comply.
Companies will either charge more for games, or start slashing features to keep the budget down in order to make up for the increased costs. Modders will start charging more or making lower quality mods that they rush to make with less effort because they are getting less for them. Both of these directly affect the consumer. Who do they not affect? Valve. Valve, who is doing the bare-bones minimalist effort of a middleman and collecting the vast majority of the pay.
Valve is attempting to monopolize PC gaming through their service, and that is going to be bad for EVERYONE. Except Valve, of course.
When this happens, don't say you weren't warned. Don't act as shocked as people were when Amazon began extorting book publishers to pay them or they wouldn't get their titles carried. Monopolies are bad. Period.