Transpiling is more about broad browser compatibility than anything else. In any event, even if you invented some more efficient browser language, it would like

Transpiling is more about broad browser compatibility than anything else. In any event, even if you invented some more efficient browser language, it would likely need a universal browser-compatible compilation target anyway (like web assembly). This isn't particularly a step forwards from TS compilation to JS, and the loss of all common JS libraries would be a huge drag on adoption.