Also, Servo is just the engine. And modern web render engines are themselves highly modular. I think the Gecko engine powering Firefox have had its javascript i

Also, Servo is just the engine. And modern web render engines are themselves highly modular. I think the Gecko engine powering Firefox have had its javascript interpreter replaced 2-3 times. So when it comes to it, the most likely outcome will be a kind of "my grandfather's axe" scenario where over time parts of Servo replace Gecko within Firefox until Servo has completely replaced Gecko.