I believe this is also the same approach Mozilla have taken with Rust & Servo (although Servo is not quite as production-ready as I was expecting this long down the line, although re-reading the description, perhaps it was always meant to be a cute project, not a browser for the masses)
I think the history of Servo has been retconned a bit. See for example this fluff piece from late 2012, shortly after the project was publicly announced:
"A new rendering engine for Firefox is being developed by Mozilla, and it’s being built from the ground up for multi-core computing environments. Mozilla has delivered some pretty major performance improvements to Firefox over the last few versions. The IonMonkey JavaScript engine is firing on all cylinders, and now it looks as though the Gecko engine may soon be replaced by something much more modern."
If this wasn't Mozilla's intention from the beginning, they certainly didn't do much to temper this enthusiasm or clear up the misunderstanding.
