I don't think it's a loss, I wouldn't mind if every browser switched to Webkit. Problems with cross-browser rendering differences would be gone, all browsers would use the same newest technologies and the engine would evolve much faster.
For the same reason, this also leads to degradation, and slows down the evolution of the engine. A good example is gcc vs clang. For years gcc was the compiler for *nix, then clang comes along. This led to a new found enthusiasm to make parts of gcc better. The same happened for firefox when chrome came along. and works the other way. We even have half-decent IE releases now.
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.
My point is that it's bad if all browsers switch to WebKit and web sites absolutely require WebKit to work correctly, which is what the parent of my comment was suggesting should happen. This point is somewhat independent of what happens with Presto per se.
It's not as if the browser vendors simply do a 'git pull && make' every couple of weeks.
But your point is taken.