This is exactly the reason why Typescript took off and none of the other compile-to-JS languages did (other than a brief flash by CoffeeScript). Dart had a very similar problem as Reason - it wasn't Javascript. The interop between Dart and JS libraries was just too much of a pain to deal with, where in Typescript everything just worked.
Since building your own ecosystem that rivals the JS ecosystem for libraries is an extremely difficult task, any candidate to replace JS must have good interop with its libraries to be successful.
This is exactly the reason why Typescript took off and none of the other compile-to-JS languages did (other than a brief flash by CoffeeScript). Dart had a very similar problem as Reason - it wasn't Javascript. The interop between Dart and JS libraries was just too much of a pain to deal with, where in Typescript everything just worked.
Since building your own ecosystem that rivals the JS ecosystem for libraries is an extremely difficult task, any candidate to replace JS must have good interop with its libraries to be successful.