I'm of 2 minds on this.
On the Valve side, their rationale is related to scale. I guarantee the costs to administer the first million dollars in sales as a percentage is higher than the subsequent million, and so on. So in that sense, cutting fees at high volumes is fair to the developer.
On the Epic side, they're trying to incentivize the little guy using their development platform and marketplace, with the idea that if a small dev hits a home run, Epic will reap the benefits of high sales. It doesn't hurt them too much because most small devs have low sales.
2 very different philosophies resulting from competition that incentivized experimentation. This is how the free market is supposed to work, happy to see it functioning a little bit.
