If you do a risky project that might fail or not pay off, and you don't want to get bad PR, you can do only two things: Keep it entirely secret, or develop it in the open but make no promises whatsoever. For an open source company, the choice is pretty easy.
I always thought it was a public secret that of course they're going to put it in the browser, but they don't dare promising that before they know it's going to work out. In fact, I wouldn't be surprised if nextgen Firefox is just going to be Servo + browser.html.
The reason why you can't find an answer as to whether Servo will replace Gecko is, in fact, that there is no answer to that right now. Any sort of wild speculation as to whether Servo is going to replace Gecko is extremely premature at this point.
Such is detailed at Servo's roadmap. https://github.com/servo/servo/wiki/Roadmap
Specficially:
>> Internally committed 2016 goals
>> Ship one Rust component in Firefox Nightly, riding the trains
>> Experiment with the uplift of a major piece of Servo into Gecko
Bugs to watch:
- https://bugzil.la/1135640 - (oxidation) Add rust compiler support
- https://bugzil.la/1243581 - (stylo) implement support for Servo's style system in Gecko
- https://bugzil.la/1151899 - Integrate the rust-url parser into necko