From my point of view the next thing that fills that role would be unity. Though a big part of unitys strength is not being bound to one platform (web) but instead being cross-platform (web, mobile app, desktop, ...). Iirc it has been used by artists for both "advanced" topics like movies as well as for small games.
I am not sure if Unity will ever fill the gap left by Flash.
Flash IDE was easy to use even for very novice creators (at least at a basic level, not talking about ActionScript). So there was a ton of simple animations, silly games, just weird experiments without any purpose or meaning. Many people were just having fun with it and sharing their creations.
When Flash died, this whole ecosystem somehow disappeared instead of migrating to a clear successor.
I might also be getting the timing of it all wrong. Perhaps when Flash was slowly being deprecated it wasn’t that popular anymore and by that time most creators moved on to different—more fractured?—ecosystems.
