The obvious response to this is that the non-techy family members wouldn't be able to use it, but honestly most home entertainment setups are already crazy complicated, whereas everyone knows how to use a PC (for now)
We also have an old laptop attached to the TV. We set that up in the lockdowns so we could use a webcam on the TV and a wired microphone on the coffee table to "get together" with friends and family, still use it occasionally for Dungeons & Dragons with friends who live too far away to visit often. The Apple TV doesn't support webcams, but wins at everything else, hands down. Even for desktop-y stuff, streaming my Macbook or my girlfriend's iPad to the Apple TV is less hassle.
Desktop ergonomics just don't work on the couch, at least for us, even with a nice-ish wireless keyboard with touchpad. Having a touchpad remote with just four buttons that have very predictable functions and a simple mobile-ish UI is nice, even to me, and I'm a desktop power user otherwise. Desktop OSes are for work, school or uni, most people aren't inclined, encouraged and/or enabled to explore and play in those, so they don't get them the way desktop power users do and tend to expect everyone else to, or the way people get mobile UX.
If you want something nearly everyone can pick up quickly, even older children and some seniors, make it touch-based, responsive, give it proper apps and the same core animations mobile phones have and you're 80% there.