It would have been hard for us to get the real-world battle testing needed to get Servo's style system into shape without rolling it out in Gecko.

https://github.com/servo/servo/pull/16971 is an example of Bobby Holley, a Gecko developer (who's awesome, by the way) landing almost a full rewrite of the multicore scheduler, making tons of sites faster in Servo.

> they don't seem to want to just rip-out Gecko and put Servo in there

It's hard enough to change the wheels of a car while driving on the highway, you really don't want to do all four at once.

Car analogies (and generalizations in general) are not so useful.

I've seen many cases (and been frustrated) working in a corporation where high levels of caution end up creating much more total work to make changes incremental. And when you do finish, the code is often not is nice as if it had been all done at once (at which point it probably gets left as is as more important things than refactoring, in the eye of business people, come along).