Key sentence in their justification seems to be:

"allowing this enables many common JavaScript patterns"

Honestly at this point they should make a new strict "no js" mode, as the ecosystem likely has reached a tipping point where you can get by without mixing typed and untyped js at compile time. Wonder if targeting wasm directly would help ensure those boundaries are ensured...

https://www.assemblyscript.org/ is pretty darn close, but it would probably be better to converge and not split communities