I am sure any Modern Smartphone will be fine and consumers have different needs which will determine the type of device we have and how often you need to upgrade. For the flagships though that is likely to be fairly regularly , however being a model behind should not be a significant issue. Whilst many will purchase new devices as soon as they are available , whilst others will wait until there is a better deal someway down the line. However purchasing late means it may soon be superseeded (however that is fine if it suits your requirements)
Commercial factors mean that companies need to bring new devices out on a regular basis and entice us with new features , however some of this will be demand lead as consumers want more from their mobile devices and technology to enhance their productivity/entertainment. Certainly technology can never stand still for too long , we need to think of new ways devices can enrich our lives (of course our own affordability plays a part in this)
As far as the S21 goes somw would argue the S20’s screen resolution is more impressive at1440 x 3200, or Quad HD, whist the Galaxy S21 is 1080 x 2400- Full Hd+. However the S20 can only use QHD resolution or the 120Hz refresh rate(not both at the same time) Only the S21 Ultra has QHD resolution but can use 120Hz simultaneously. See a report here : https://www.techradar.com/uk/news/samsung-galaxy-s21-vs-galaxy-s20-comparing-flagship-samsung-phones
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .