we have HTML5 now which is better than Flash ever was

In some ways, sure. In others?

Trying to download HTML5 games sucks (there is no container format). Trying to play them locally sucks too (simple http server). If preservation efforts here can match how it has gone for Flash content, it'll likely be only thanks to web crawlers people are using now.

Vector graphics was a huge technical feature that (even if still technically possible) has been largely abandoned. Even ignoring visual style, it's less data to load (esp. w/simpler stuff). Particularly for animations (even a 1min18s clip is ~5x larger when rasterized), it seems silly to me that Google didn't attempt some sort of HTML5 vector video support (for an extreme example, see the 10-hour homestar runner complilations on YT) which could likely also be used (at least partially) for digital presentations. Vector being rendered natively at runtime means content creators(/platforms) aren't required to export/store videos in multiple resolutions (which for individuals, might just mean not supporting the higher ones).

Also, WebGL errors and Unity DRM making it even worse, though I'm not sure how much those are still present. Personally I have lost WebGL game data (unclear why, thankfully this isn't always important) more often than I ever remember with Flash.

Yeah I acknowledge modern web is superior to flash in another comment, but argue the accessability has gone down. Also I was not 'growing up with it,' I was getting paid to write actionscript, then as3, then as3 inside of adobe flex/eclipse.