[jeswin]

>I think the sunk costs are too large but I'd love to see Valve fork UE4 and put their engineering might next to Epic's. Would love to hear why this thought of mine is naive/misguided.

UE4 source code isn't open source (to fork), though the code is available to look at. One could reimplement it, but the engineering effort will be monstrous.

[adventured]

You can in fact fork it. You can do almost anything you want to with it, so long as you obey the EULA and pay any royalties owed.

https://www.unrealengine.com/faq

"What modifications can I make to the source code?"

"You can extend it, modify it, fork it, or integrate it with other software or libraries, with one exception: You can’t combine the Unreal Engine code with code covered by a “Copyleft” license agreement which would directly or indirectly require the Unreal Engine to be governed by terms other than the EULA."