But what about compilation speed? Hot reload? Maintainability? What is the bus factor for these kind of technologies?

Yea, its cool to have strict type safety, but what's the point of it doesn't have all the benefits from other Frameworks that took years to polish

JaneStreet has enough free cash to not worry about those issues. If one of their key persons dies in service, they can go out and contract the worlds leading expert in that domain, and their annualised rate isn’t even a rounding error on their bottom line.