So, for Servo to catch up to the current state of Firefox and to then also keep pace with new web standards, it would have to be developed much quicker than Firefox is being developed. So, that would require more than double the development capacity that Mozilla currently has, as they can't drop Firefox development either.
There is some points that could alleviate this: <ul> <li>A smaller project grows quicker, <li>it being written in Rust might speed up development in the long run and <li>several components are now shared between Firefox and Servo, meaning that the development and maintenance work is shared as well, <ul> but it still is far away from realistically completable.
Which is also not what the project is meant to be. It's a research project. Trying out new things, creating components that can be used in Firefox, and doing experiments with VR or similar are exactly what it can be used for.