You can ship GLES games on desktops no problem. And anything remotely notebook / desktop class graphics wise is so much more powerful than some Mali GPU that you can ignore "performance characteristics" for the most part and assume brute force will be fine. If your game runs on a feature phone, it will run with GLES on desktops.
What I was getting at with the performance characteristics comment - though don't ask me why I didn't act like normal people and just say this, because I've no idea - was that the game will probably look crap on a PC. That means you'll have to spend some time making it look a bit nicer, if it's to be competitive. This is all going to involve a bit more than a simple button press.
Mesa doesn't do that:
It does, every driver independently advertises GLES 3, and while it doesn't support full 4.3, every driver supports GLES 3. That post is like three years old now.
Game engines, as every similar kind of middleware, are an abstraction that is almost always leaky. It's still a great improvement over doing everything natively, of course.
What. Why do people think that? Even if you use Unity, the DirectX/OpenGL distinction still matters, not to mention all other platform-specific stuff. Of course, with Unity you _can_ develop a very simple and unoptimised game that would be able to compile on all desktop and mobile platforms with one click — but when you start to dig down and try to do things effectively, you end up with a lot of platform-specific code.