I just now finished upgrading to Big Sur, and so far it's fine, with minor visual changes noted. I asked myself a couple of rhetorical questions: why did I do this, and why did Apple do this? For myself, I think curiosity is the best answer. For Apple, I did a bit of web research and came up with the following answer for Big Sur:

"MacOS Big Sur comes with a simpler design, new app icons, smoother animations, refreshed app design for all pre-installed apps. MacOS Big Sur also has a new menu bar as well. The Mac will now also have a control centre, like iPhones."

On the menu bar, which is now black with white lettering, I noticed a new icon, and clicked it. It opens up a control center, which gives one the option to do a few things like change screen brightness, sound level, etc. At first glance it doesn't appear to be particularly useful, but I'll see as time goes on.
My overall impression? Meh! .

Jim Hamm