Flash evolved into Adobe Animate, which is used today for many television shows:

https://www.adobe.com/products/animate.html

https://en.wikipedia.org/wiki/List_of_Flash_animated_televis...

Disclaimer: I work for Adobe, but not on Animate.

I doubt open sourcing the player would hurt that revenue stream.

Good question. Why not open source?

All it takes is someone (or group) interested enough to expend the resources necessary to re-implement it. But browsers are not going to support binary extensions. So an implementation might be in WebAssembly / JavaScript -- but then Flash becomes just a translation or transpile layer onto existing web technology.

If nobody implements an open source Flash, that is the proof that it is not sufficiently interesting to anyone with resources to build it.

I suspect the Flash runtime has a bunch of patent-encumbered, licensed software and codecs and the effort to secure those licenses for open-source release is too much effort/money.

I suspect something like that. The video codecs it uses are Sorenson Spark (ancient), VP6 (google opened this), and h264 (certainly encumbered for a while longer). Though, patents haven't stopped open source projects like LAME or x264. At the very least, they could release the source for the code from 2000 which is now past the patent expiration dates.