BrightScript does suck, though—but it also already exists, and I doubt maintaining it's all that hard. Used to work on a dual-platform mobile app engine that was probably on par, difficulty-wise, with maintaining & developing BrightScript—that only needed like 3.5 devs, and not at coastal US wages.
Meanwhile, the Big Boys get a C++(?) API, as I understand it. I assume that's why some of them behave weird or are glitchy, but do look a lot like the same apps on other platforms.
[EDIT] However, they could probably do a good job making a comparably-snappy UI with JavaScript, as long as they didn't drag HTML and CSS along for the ride. Kinda like the JavaScript API on tvOS.