Nowadays with WASM and WebGL you can do basically everything in a web browser, but you'll take a 100x performance hit.

Every game using Unity/Unreal + WASM needs to download 10-20 MB of JavaScript before it even starts loading the actual assets. With Flash, that could have been a 10 KB SWF instead.

Similarly, Flash had really fluid animations that were drawn purely on the CPU. Why my 266 MHz P2 could have better vector animations than people nowadays can squeeze out of a 3070 with WebGL is beyond me.

In my opinion, we still DO NOT have any worthwhile replacement for the ease of cross-platform development combined with excellent performance that Flash had.