Lumberyard is essentially the CryEngine Engine that Amazon bought. It is more complex but it also shipped excellent games on it. I think game engines should always be built around games shipped. Unreal is quite nice in that aspect. Yes the engine becomes more tuned to the game types that you ship, but many of the pain points are smoothed.

I'd love a game engine platform that would come along and promise API/library stability for 5 years. Any game you built on that engine will be able to run 5 years later if you need to update it.

There isn't a ton that changes with game development surface level, the subsystems change quite a bit, that is fine. That is what the engine is for, to simplify the subsystems and create a stable platform on top to speed up development, not have a bunch of leaky abstractions and new unstable platforms on top of that.

I wish that game engines had more standards that were industry respected and you could bounce between engines more easily as well, like other frameworks do with standards.

We are in a "move fast and break things" age and I can't wait until it gets back to "let's move fast but not break things if we can help it unless it makes massive improvements that make it worth the cost/time and most importantly, maintenance tax on updates that aren't needed or break things for no reason. And when we do break things we will do it in a way that is parallel and if possible just swaps out the subsystem not requiring massive surface level changes that just get you back to baseline functionality".