They do a bit more than this. They do streaming and transcoding for a start, that's fairly essential for a functioning video platform. They also have native apps for a range of platforms. Support subtitles, multiple audio tracks, all sorts of codecs. And then there's all the metadata stuff as well, getting blurbs, posters, ratings, categorisations, episode naming (which has so many awkward edge cases) etc all from various metadata services. Oh and they do this all with multiple users and ACLs.

Plex also then adds social features and curation. The value of that to its users is debatable, but it's additional complexity regardless.

These are pretty big projects. A founder moving on is a fairly big deal.