Even if the degradation of the user interfaces is noticed especially by older people, I doubt that this has anything to do with them being old and I believe that it is caused only by them being more experienced, i.e. having seen more alternatives for user interfaces.
For several decades, I have used hundreds of different computers, from IBM mainframes, DEC minicomputers and early PCs with Intel 8080 or Motorola MC6800 until the latest computers with AMD Zen 5 or Intel Arrow Lake. I have used a variety of operating systems and user interfaces.
During the first decades, there has been a continuous and obvious improvement in user interfaces, so I never had any hesitation to switch to a new program with a completely different user interface for the same application, even every year or every few months, whenever such a change resulted in better results and productivity.
Nevertheless, an optimum seems to have been reached around 20 years ago, and since then more often than not I see only worse interfaces that make harder to do what was simpler previously, so there is no incentive for an "upgrade".
Therefore I indeed customize my GUIs in Linux to a mode that resembles much more older Windows or MacOS than their recent versions and which prioritizes instant responses and minimum distractions over the coolest look.
In the rare occasions when I find a program that does something in a better way than what I am using, I still switch immediately to it, no matter how different it may be in comparison with what I am familiar, so conservatism has nothing to do with preferring the older GUIs.