The thing is that rewriting a browser engine doesn't just happen over night. No bigger browser engine has been written from scratch since the previous millennium. Also, no browser engine implements all currently specified webstandards. So, Mozilla would have to full-pelt develop Gecko to be able to keep up with Blink in webstandards, and then also develop Servo at more than full-pelt to be able to catch up in finite time.
That just doesn't add up, which is why Servo was specifically started as a research project. It might never be completed. They have been able to integrate various components from Servo directly into Gecko, therefore sharing the development work which they didn't really plan with, so that's one tiny reason to still hope for it to ever catch up, but yeah, just don't be too optimistic.
Having said all that, the integration of components from Servo into Gecko isn't done yet. WebRender is still missing, which should bring another good performance boost. It's sort of been overhyped with artificially complicated CSS animations [0], so the real world effect isn't that big, but it's still very much noticeable.
[0]: https://www.youtube.com/watch?v=u0hYIRQRiws
Aside from that, on Android, Mozilla is building a new framework for building Android browsers in general, called "android-components", which also includes a much cleaner integration of Gecko on Android, called "GeckoView". They've already rebased Firefox Focus onto GeckoView and are actively working on rebasing Firefox for Android (Fennec) onto it, too, which is internally called "Fenix". This should also significantly reduce the lagginess and speed up their development in the long-run.
I guess, what I'm mainly trying to say is that there's definitely still things happening.