I chuckled a bit while reading this, because what you wrote is exactly where Blink came from. It was a fork of webkit, which in turn was derived from KHTML. Then again, the fact KHTML was discontinued does support your point to an extent too, I guess.

But the point is, Chrome is doing exactly this - providing the engine free as in beer and letting people embed it however they like. And yet, what you’re predicting, ie. not using the original but just using forks instead, doesn’t seem to be happening with Chrome - they still enjoy a massive fraction of the market share. There’s no reason to believe that this couldn’t happen at Mozilla as well. People usually want the original product, and it’s only a small fraction of people that are really interested in using the derivatives.