Languages like this don't make sense anymore. You can just pick a mature, feature-rich language you like (Rust, Kotlin, Python, probably many more) and transpile to JS.
If you’re interested in targeting JavaScript, languages like this are your only reasonable alternative to JavaScript itself. (If you’re interested in targeting the web more generally, WebAssembly is a better target.)
If you don’t mind I guess you could just render to a canvas but then you lose a ton of functionality provided by the browser. (Eg: making a responsive page in WASM seems hard)
It seems to me for most web applications JS is the only real alternative as of now
It’s completely fine. At present you’ll need to trampoline via JS to make the actual DOM calls, but that’s not typically a performance bottleneck; and such bindings can give you exactly the same API, if you choose.
Yes, work is just about done in browsers for giving you direct access to these things without the JS trampolining, and lots of people are hanging out for that to be dependable, but in practical terms it changes nothing. Nothing. No change in expressiveness; it only gets you probably slightly better performance, and a simpler technique for binding (which is typically immaterial for end users, as libraries were handling that for them).
> Rust began as a personal project in 2006 by Mozilla Research employee Graydon Hoare, named after the group of fungi that are "over-engineered for survival". Mozilla began sponsoring the project in 2009, and would employ a dozen engineers to work on it full time over the next ten years
So by 2013 it was already 7 years old. I very much doubt the original personal project was viewed as "a lot of sense" etc.
B) What are the downsides?
For the languages that target wasm instead, there are different debugging stories. Kotlin's is very good, Rust's is pretty immature.
Source maps work great for TS because it is just "JavaScript with types" at this point.