That was a long winded way of saying you don't know what Jellyfin actually is or does. Mpv is a client, it only fills the role that the various Jellyfin clients perform and a better comparison would be against a heavier weight media player such as Kodi.
What you suggest works well enough if you have a reliable network link to share CIFS or NFS over, but what do you do when away from home on a rubbish link that doesn't have bandwidth to stream all your high quality bluray rips? You want transcoding in that situation.
Also, I'm a seasoned Unix sysadmin who knows his way around the cli and I can say with certainty this isn't for people who know the cli, it's for people who just want to prove you can do anything from the cli even if suboptimal.
They run their own full OS these days on under powered SoCs to cut costs. All so they can claim to be a "smart" TV until of course the flow of updates stops a year after manufacture and all the apps stop working. Then it's back to being dumb as well as being a massive security hole on your network.