I've worked on a few "high end" games, and used to work for Epic.
Epic dogfoods the engine with _real_ games. Fortnite, Paragon, Battle Breakers, Robo Recall are all titles written in UE4, by Epic, which have had active contributions to the engine in their development. Whether you like the game or not doesn't have much of a bearing on the impact the game has on the engine.
They did the same with older games too, and the remnants of games like UT and GoW are very visible in the source of the engine itself.
Unity, generally makes money by convincing more people that they could make video games, and keeping up that belief so they keep paying for versions or subscription fees.
Epic only makes money after games have actually shipped.
That's not actually true, the royalty only license for Unreal only started with UE4, and there is still "contact us" licensing available too.
On the unity side, an increasing amount of their revenue comes from "grow solutions" [0] which is basically their ad networks. Game engine development is a funny business
It's not exactly true, but the incentives still align in practice.
"Contact Us" Licensing has always existed as you state, but has often included the possibility of royalties, and even when it doesn't it's always been at the price ranges where shipping a game was more likely than not.
And in-fact, a problem with early Unreal licensing has been Epic's approach to support of "If it's not a problem in an Epic game, it's not our problem" Which has improved dramatically since changing their licensing model.
And regarding Unity, yes, their ad networks are becoming more important, but that still doesn't do as good a job at aligning incentives, but more importantly, Unity has 15+ years of development in a model with unaligned incentives, which when you're talking about technical decisions, those are really important.
And to be clear, I'm not arguing against the point that Epic being a game developer first is an important factor, I think it very much is, however I'm suggesting there are also other contributing factors.
Epic dogfoods the engine with _real_ games. Fortnite, Paragon, Battle Breakers, Robo Recall are all titles written in UE4, by Epic, which have had active contributions to the engine in their development. Whether you like the game or not doesn't have much of a bearing on the impact the game has on the engine.
They did the same with older games too, and the remnants of games like UT and GoW are very visible in the source of the engine itself.