Your argument seems more like one against transpilers in general. Contracts are useful during development, they're typically stripped out in production builds,

Your argument seems more like one against transpilers in general. Contracts are useful during development, they're typically stripped out in production builds, so there's no performance overhead there. If you find a bug in production, turn on your contracts and you'll quickly discover the source of the error. Contracts can validate the state of the world at runtime in a way that can never be covered by unit tests, unless your tests cover all possible inputs to your program. It doesn't mean you don't need unit tests too, they are yet another layer of safety. I use this in combination with Flow, which means I have to transpile regardless, so the overhead is very low for me. Of course I'd like them to be part of the language proper, but for that to happen there needs to be developer demand first.