Servo wasn't intended to build a full engine to replace Gecko, just to prove out components (Stylo, WebRender, etc) that could be integrated back into Gecko proper, built in a way that takes full advantage of Rusts strengths, and it's been pretty successful by that measure, IMO.

There was plenty more work to do (other commenters have mentioned a new layout engine), and I'm disappointed to see Mozilla abandoning such a core next-gen R&D project that has already brought substantial improvements to their main product.