Well, you have a point but you see, both Rust and Servo are at the nascent stage. We still have to see widespread adoption of  Rust. Servo is definitely way behind any browser engine in terms of features and specs and rewriting from scratch might also introduce new bugs(happens with every software product of considerable size). To summarise, both of these are very nice and ambitious goals but I really hope they turn into something nice rather than just an ambitious project(Servo).