I have written about this on some threads every now and then, but I wanted to gather some thoughts about why I haven't, and likely never will, upgrade to 4K Blu-ray.
1. Regular Blu-ray is good enough. Honestly, I still wonder often at how good the picture quality is on most of the Blu-rays I watch, with both old and new movies and TV shows. The last thing on my mind is "I wish the picture was better" when I watch Blu-rays. What I've seen of 4K Blu-ray at my friend's place, it looks definitely nice, but I honestly couldn't see much, if any difference to a regular 1080p Blu-ray. It was on a 75" 4K TV with a watching distance of maybe 2-2,5 meters.
2. I really don't feel like upgrading my Blu-ray collection. I upgraded to regular Blu-ray in the early 2010s. Luckily my DVD collection wasn't that big at the time. I sold most of my DVDs little by little when I upgraded them to BD and I was able to sell them over a couple years for a pretty good amount of money, so I luckily didn't take that much of a loss with them. However, my BD collection quickly got much bigger than my DVD collection ever was and I have a lot of really cool collector's editions and steelbooks and I definitely don't want to sell or upgrade them. Besides, I think most of the 4K steelbooks are a lot lamer in terms of design than their original regular BD steelbook counterparts.
3. Not upgrading is also my way of telling the studios that enough is enough with buying the same films (or TV shows) over and over again. If everyone upgraded to the 4K Blu-ray as quickly as the studios hoped, there would probably be a new format in like 3-4 years from now. 8K, 10K, whatever. Just attach a new marketing gimmick to it and it's easy to market as a revolutionary technology. "Super ultra resolution, hyper ultra clear 3D audio" etc. Many have speculated that the 4K Blu-ray will likely be the last new format of physical media due to the rise of streaming services and it's likely true. However, if 4K had sold as well as the studios hoped, a new format would likely have risen at some point in the near future. 8K TVs are already being sold.
4. Equipment. I bought an expensive premium BD player a few years ago by Panasonic that cost about 400 euros. I don't want to ditch that and buy another one, as it is still basically like new, works like a charm and it's just an overall great player.