The purpose of 4K HDR is to maintain that quality, otherwise it is not worth it to downscale imo
You are conflating 4K resolution with the important technologies of HDR and WCG. Marketers knew that these technologies wouldn't "sell" to the average joe without significant explanation. But everyone "thinks" they understand resolution so that's the easy sell.
What is the purpose of down-scaling? Sure you gain performance, but you lose the quality of 4K, so why not get your content in 1080p and do native 1080p and you can push 3X for example or 4x with RIFE
When I watch 4K HDR content on my PC, I don't see any detail loss at all, even on a very large screen. So whatever loss there is, isn't significant or obvious without pixel peeping. I can also download 1080p HDR rips that look as good as many 4K discs. That's why I decided to re-encode most of my non Dolby Vision 4K discs down from 60GB+ to half or less, saving disc space while keeping 99% of picture quality.
As for my blu-ray collection, I literally can't see any quality loss at all, and for blu-ray, as long as you are using a decent player then you shouldn't either. 4K HDR playback on a PC is a little trickier, not because of the resolution, but because of HDR. Especially Dolby Vision and HDR10+