Because the more effort put into improving JavaScript performance, the more entrenched something terrible yet ubiquitous becomes, and the less chance we get of replacing it with something that is not a cyclopean horror?

Personally, I think its a good thing that JS gets SIMD support -- since Dart has it, and it would be better if the discussion over whether a new standard language to replace JS was needed was over language fundamentals, not one competitor or the other missing useful features that aren't part of the fundamental language structure.

Plus, Dart will be able to use SIMD support when compiling to JavaScript :)

I don't think it's reasonable to avoid improving something because it might then be less likely to be replaced. We do need improvements right now.

The only way around the problem you raise is to make something truly better, and convince people to use it instead.

Haha wow. It's just a programming language. == vs === got you tripped up?