It's a "gigantic win" for everyone in the same manner that Uber paying drivers and customers (via bonuses / free rides) to use their service was one for everyone involved . Sooner or later, Epic will have to stop doing this once they get their market share and there is no guarantee that this will be the financial model they follow (free games frequently, paying devs to be exclusive etc).

It works in a different context ( platform exclusives ) because the lock in / costs to developing for a different platform are far different from that for different stores on a single platform.