Valve will have to start giving money back to developers too, or they'll lose them to Epic. Either by reducing their % cut of sales or exclusivity cash or whatever else. That's the definition of competition.
That competition will be a race to the bottom as far as consumer choice and options go, perhaps Valve has enough revenue models via the marketplace to make up for a reduction to their sales cut, but that cut is partly due to how barebones the epic store service is, where we have to wait over 6 months for a shopping cart feature to be implemented. I don't see the developers selling freely generated keys to alternative markets industry surviving if Valve has to compete against epic in exclusivity, which again would be a hit to consumer choice and options.