What speculation? If you have a "let" in some lexical scope, AND this is strict mode, AND there are no calls to eval in that scope, AND there are no assignments within that scope... how is that different from const?

I can understand why in a JavaScript engine there may be plenty of other concerns and it may be completely reasonable to not do the analysis, but in the common case it should (at least) be something you could do just by walking the AST, if you so desired, without even knowing the surrounding code.