Servo is a prototype – closest to your "Pie-in-the-sky" option.
I'm not affiliated with Mozilla or Servo but from the information on the Servo page and the number of Acid2 and other issues they're tracking, it's clear that there's still plenty of work to do. Throw in the fact that the Rust language itself is targeting the end of the year before they hit version 1.0 and you'd have to guess that Servo as a separate project likely has minimum 6 months (more likely a year) before they examine whether it's successful enough to start integrating with Firefox.
Then they'd need to integrate and test.
Think about how long the new "Australis" UI was in development (more than 2 years). It was just a user-interface change without changing programming language or other dev-tools.
The renderer is the core of the program. And integrating Servo would involve integrating and testing a new language along with the new code. I doubt a large, capable team could perform that much integration and testing in under 12 months – even after Servo itself was considered "complete" (which it isn't).
My prediction: a release of Firefox with Servo code is 2 years away or more (assuming Servo is considered a "success" in 6-12 months).