Flash is still a big loss imho, the ecosystem of games, movies and demonstration thingies was amazing and they were accessible to create by many. Unlike Java applets that slowed the main browser UI thread to a crawl if they didn't load they usually didn't), Flash didn't have such slowdowns.
One exception is early 2000s Runescape: that was Java in browser but always loaded, no gray screen and hanging browser. They knew what they were doing.
Many of the old games and movies still play back well with Ruffle installed (https://ruffle.rs/). Newgrounds embeds it by default for old interactive flash media that they couldn't convert directly to video.
It's not a perfect fit, but it works. The speed of Ruffle loading on a page is similar to that of Flash initializing, so you can arguably still make flash websites and animations to get the old look and feel if you stick to the Ruffle compatibility range. The half-to-one-second page freeze that was the norm now feels wrong, though, so maybe it's not the best idea to put Flash components everywhere like we used to do.
Runescape proved that Java could be a pretty decent system, but so many inexperienced/bad Java developers killed the ecosystem. The same is true on the backend, where Java still suffers from the reputation the Java 7 monolithic mega projects left behind.
It's good that we have the runtime to run old Flash games. What we lost is an extremely easy environment for authoring/creating them. Nothing has come even close since Flash. Not just game, but any kind of interactions and animations on the web.
I think perhaps what was lost is mostly this: Macromedia. They had a knack for making content creation simple. Flash was just one of the results of this: It let people create seemingly-performant, potentially-interactive content that ran almost universally on the end-user computers of the time -- and do it with relative ease because the creation tools existed and were approachable.
Macromedia also provided direction, focus, and marketing; more of the things that allowed Flash to reach saturation.
Someone could certainly come up with an open JS stack that accomplishes some of the same things in a browser on a modern pocket supercomputer. And countless people certainly have.
But without forces like marketing to drive cohesion, simplicity, and adoption then none of them can reach similar saturation.
One exception is early 2000s Runescape: that was Java in browser but always loaded, no gray screen and hanging browser. They knew what they were doing.