I think it depends on your setup - if you've got a good 4k HDR TV then by all means you could just watch then delete and it would be worth it. But yeah good point, may as well do 1080 otherwise, if you want a collection. I've only got 90 movies at 1080p and struggle to justify keeping more than that.

I find the best way if you're on a budget is to have a small collection of 4k movies, with an even smaller rotation of new 4k movies - then have everything else at 1080p x265. Still want at least 8TB ideally, so down the NAS rabbit hole we go…