There is no content for 5k yet.
YouTube & Netflix have only limited 4k. So why the need for 5k? I'm thinking that it would be unused unless you're doing photo editing. Other than that it appears to be a waste. Logically you would want a 8k display as the next jump for future proofing. I have a 4k Samsung TV which is outstanding! But 5k hits me as odd. (Pun intended). Why did Apple go that route?
Well, for your tv you probably sit so far away 720p would be adequate. And like you said, 4k is limited. So isn't your tv an overkill? Yeah it looks good if you walk up to it, but is that really needed?