That's a good point, and it's one that isn't solved yet in the foss space.
There are some success stories like Blender, and other projects like Thunderbird and KDE who have recently made their model work through voluntary donations, albeit by hiring competent management of such donations. And there are lots and lots of projects somewhere in between.
The interesting questions to me aren't so much about Plex, but the infrastructure behind all the tools we use: NTP on Linux, build tools, ffmpeg libraries, etc. Lots of other companies make products that make money, yet kick back nothing to these.
Would a royalty system work? I dont know.
Lolol yes, elastic was a pig for me too