> Well, you can always modify source code and use it privately - you hardly need a license for that.

You do need a license to modify the source, use it internally, and then sell a binary you've produced with the modified source.

It's the "sell a binary" bit that introduces the need for a license.

The point is that if you're a game company. You have a way to legally make modifications to the engine even if Epic goes belly up.

No you cant.

Who could stop you and how would they know about it?