Unfortunately for Unity, they don't make any more money on a hit game than on a big-budget failure. All that matters is how many seat licenses they sell. Selling developer tools is not a great business.
The Unreal Engine has an interesting license. There is no upfront cost but there is a 5% royalty that starts after the first $3,000 of revenue per product per quarter. Which is great for smaller shops, but seems like it would scare away the larger organizations.
Larger orgs never use that license. They all negotiate privately and secure a license for a fixed cost and no rev share. IIRC the cost for a AAA game Is around a few hundred thousand.
Unreal Engine is the most used Game Engine in the industry (not counting indie/mobile games w/ Unity), especially for AAA games. HeartBreak shared the list. For professional development the Unreal Engine makes it possible to make amazing games with a small team of programmers, iteration time is fast making development a lot faster than before or trying to use a custom engine without well developed tools.
One reason for this is because a game engine is so huge and complex that they have a massive learning curve, much more so than any web framework.
If you use Unreal Engine / Unity, you have a pretty big pool of talent that is already proficient with the engine and can hit the ground running, rather than having weeks of learning the intricacies of your specific game engine.
Epic still offer completely upfront licensing with no revenue splitting, although the price isn't published. UE3 was supposedly in the $500k range so I imagine UE4 is similar.
But, seeing a breakout hit made with Unity is very likely to make a lot of devs think, "I know, I'll use Unity!" I'm confident there's gonna be a big sales bump from the success of Pokemon Go.
I have no proof for this but I'd guess they are making a lot of their money from their video ad network. By making it super easy to integrate into mobile games built with the engine they get a lot of ad publishers on board easily and can then sell ad space.
I've been playing with it a bit in the last few days, that's pretty interesting. I've got an older android phone, and it runs really smoothly for me. I'll have to give Unity another look I think.