The point of these super high resolutions is not necessarily that everyone owns a monitor at that resolution. It's simply that, in general, a downscaled high resolution image or video looks better than a 1:1 resolution. This has been my experience, too, with content like Rescraft's; I notice the difference especially around edges. Supersampling, which is basically the same thing, has long been a staple in games.
Now, of course, there is a lot more detail to this topic (pun intended), but calling 4K a "waste of storage space" is an exaggeration. Especially when storage, nowadays, costs 1,2 cents per GB. So, unless you are actually dirt poor - which is possible - or your hardware can't run a video at higher resolutions - also possible - choosing 720p over 4K or even 1080p seems to me a choice for quantity over quality.
To be clear, I'm running a 5700 XT and a 6600K @4.6 GHz. The only issue I've had was with Rescraft's Panam_anim_30062022, which my cpu can't handle. But if I can get it, I'll take 4K 60 fps over 2K 30 fps, any day of the week.