Flash was great, someone creating a better Flash has been long overdue. I think Rive looks like a great attempt at a new Flash.
I know Adobe renamed Flash, and I haven't talked to anyone in the cartoons space since then, but it seems like innovation/attention to the Flash toolchain died with the player.
If the goal is to render vectors I imagine it is more complicated due to quirks in how Macromedia triangulated their vectors, but someone has done it before http://blog.bwhiting.co.uk/index.php/2014/03/17/introducing-...