Is it really so unlikely? Firefox integrates all kinds of components from elsewhere; why shouldn't it continue adopting parts of Servo where appropriate?
It's one thing for a mozilla team member to say "Hey, let's pull the servo css layout engine into firefox" It's a whole different thing for someone to say "Hey, let's pull the webkit css layout engine into firefox".
The servo stuff, while for experimentation, was ultimately geared towards the notion of landing parts of it into firefox. It was built for that. Under an opensource foundation maintainer model, there's a strong possibility that it moves from that as a goal.