Because the second it's not javascript, you're going to lose 100x the audience. You'll also compete with all the other languages that can compile to Javascript and then the question is entirely different. CoffeeScript did something similar decades(?) ago when the ecosystem was vastly different and you could argue CoffeeScript provided more value - it still failed.
At the end of the day, Typescript is one way Microsoft is competing with Facebook. The only reason you want native JSX is because React won the framework wars. If Microsoft can win with Typescript, they'll have a larger say in the future of Javascript and then web development in general. They'll never move away from javascript.