Are you sure it's any better? Last time I've used a JS heavy site the UX was just utterly awful. Flash had bad bits to be sure but it could be snappy. I had hope we could do better but a lot of sites seem as bad or worse on arguably better hardware.
I'm not sure the tech stack was ever to blame...people would have eventually fixed the bad parts of flash.
> Last time I've used a JS heavy site the UX was just utterly awful.
That's an issue with the web site, not the underlying technology.
> […] people would have eventually fixed the bad parts of flash.
The Flash plugin was a closed-source component. (Various attempts were made at open-source substitutes, but none ever reached a fully usable state.) The only people who could have "fixed the bad parts of Flash" were Adobe employees, and they didn't.
I'm not sure the tech stack was ever to blame...people would have eventually fixed the bad parts of flash.