Doing this would mean being forever stuck in single-threaded land for rendering (e.g. nothing like Mozilla's Servo project could ever happen).

People have tried to parallelize WebKit and failed: too entangled, too much code making too many assumptions... Parallelism is one of those things that's just incredibly hard to bolt on after the fact.

So if you want browsers to never really scale well to large number of cores, by all means try to get them to all switch to WebKit.