XBMC forked off into Plex. Plex introduced a far better UI.
XBMC became Kodi. Kodi learned from Plex.
Jellyfin came along and learned from both of them.
So I don’t think you can really criticise Plex too much here. They were perhaps getting complacent and they’ve definitely been shown up, but they were an important step to where we are now.