I am the odd man out. I actually had a Note 20 Ultra and purchased an S21 Ultra. I also had the S20 Ultra which I traded for the Note 20 Ultra. I use a Note as intended, meaning I use the S-Pen, as opposed to a notepad at work. I generally own anywhere from 2-4 personal phones a year. Currently, I sold the Note 20 Ultra and my daily carry is the S21 Ultra. It appears I have Samsung loyalty, but I don't, I just haven't liked other brands options recently.
I thought I was sd card dependent, but now I realize I am not. I do hate that we are losing options, but I have found I don't miss it. The S21 Ultra isn't a huge improvement over the S20 Ultra or more specific the Note, but I do enjoy the phone. I took advantage of a trade-in deal with T-Mobile and was able to buy a beat up used iPhone XR for $200 to trade and received $800 in trade for it, net $600. Sold my Note 20 Ultra for $800, so to me it didn't cost me a pile so what the heck.
What slight improvements are there? I think the camera array is better than my Note 20 and much better than S20 Ultra. I am pleased there is S-Pen integration, even if not Bluetooth yet, I never used the Bluetooth gestures, as previously stated it is my notepad. The refresh rate at high resolution is barely noticeable, but it is there. I do like the aesthetics and design, but I use a case, so I can't see it, plus I could care less what others think about the phone I carry.
It is a little ridiculous to compare Samsung's sales to iPhone, because if you want to use iOS you have no other option and Androider's have so many options, therefore sales are dilute for only one brand.
I can clearly understand why folks might not have upgraded to the S21, but it didn't a great device, as is the S20 Ultra after updates and the Note 20 Ultra. You can be happy with any of these releases.