No, elmarklimov. You did nothing «wrong or illegal».
That is, most certainly, not the point.
It’s just as with pulling the trailer: nothing «wrong or illegal».
The manufacturer’s recommendation is to keep your Operating System (OS) up-to-date, by way of updates.
Going from Catalina (10.15.x) to Big Sur (11.0.x) is not an update, but an upgrade. A major upgrade.
It is like taking a car that hasn’t been pulling a trailer, and, “suddenly”, deciding to start pulling said trailer.
Just as a car can seem to be running “just fine”, even though it harbors a “borderline failure”, which can become manifest by changing its load (towing a trailer), the very same can occur with any other hardware, such as computer hardware.
You simply were not aware of the “borderline” failure.
The new “load” of a new OS, could have just as well been the new “load” of a new, “heavier”, more demanding application.
It’s no more, but, also, no less your “fault” than deciding one day to use your car to tow a trailer, that you haven’t been doing, even though the trailer is within the rating of your car (at least when in very good condition).
Apple tested this OS upgrade (Big Sur) on such very good condition MacBook Pros (and others).
Such were within the “rating” for this OS upgrade.
“Worn” systems may be on the borderline.
No one can test for all possible conditions of “wear”.