The UI that remains unchanged in a few years is perceived as old and obsolete, hence the purely marketing driven need to change it even when it works. And then there are new courses and new support contracts. Governments are a significant part of MS customer base; every time MS changes something, it's new money pouring in. As someone once said, Windows is not an operating system, it's a business platform.
They used the calculator as an example, saying that it saw several improvements but since it always looked the same, people thought nothing had changed. Improvements included things like better arithmetics, so that (10/3-3)*3=1.
People sometimes criticize the Windows calculator for perceived errors in the results even though it is a superficially simple program. What they fail to account for are the edge cases, those quirks of computation that manifest themselves for various reasons (e.g. the precision of floating point, simplifications that appear simple to a person yet are non-trivial to implement in a program). A shipping product is not the same thing as a simple calculator tutorial a novice programmer may follow.
https://www.pacifict.com/Story/
For the non-reader:
NPR Radio Piece
https://www.thisamericanlife.org/284/should-i-stay-or-should...
Google Tech Talk