Earlier quoted context omitted.
That's true, but I hate transcoding anyway so I just get HEVC when possible and play natively on Apple TV and co. (not casting).
Since you have an AppleTV, what is the advantage to you of using Plex instead of just an iTunes instance with Home Sharing turned on? Do you have non-Apple devices you're trying to stream to, or something else? I tried Plex years ago, and it wasn't to my liking because it was philosophically like Windows (Load the filename up with show information and constantly ping the internet for matches) instead of macOS (Metada…
To some extent, it's self-reinforcing. Once the FireTV gets a lead, all it has to do to maintain/extend that lead is reasonably support playback of whatever new format/source and Plex works great on it. If FireTV supported TV as well as TiVo does, it might end up with 100% of the living room display share.
(I also have Plex sharing to devices outside the house, but that's a <1% use case, mostly when it's us traveling somewhere and the kids wanting to watch something that's on Plex.)