It would be great, if you could detect SIMDable operations in classic JS (e.g. in loops) and use SIMD for interpret them. I think that adding low-level features into a high-level language is not good practice.
We will probably do that too at some point, but it won't replace explicit SIMD, just as widely-available auto vectorization support in C++ hasn't eliminated the need for explicit SIMD extensions there either.
One thing to keep in mind is that most programmers probably won't want to use this feature directly; it'll be used in libraries that expose higher-level APIs. It's still true that every feature we add increases overall clutter, but SIMD seems sufficently useful and sufficiently self-contained that it's worth the tradeoff.
