My home needs to be pretty because fundamentally I am an irrational animal motivated by my emotions. When I sit on my leather couch and look at my house plants and art that makes me feel good. When I come home and find things out of place it makes me feel bad.
A UI revamp is like coming home to a crime scene, or at least a messy kitchen.
Likewise, for many people, pretty tools make them feel better.
Of course, a UI revamp needs to be done carefully, taking existing users in account. If done well, it will be like someone living in the same home having done some cleaning.
Pretty tools don't make me feel better. Useful tools help me do work, and finishing work makes me feel better. I have suffered at the hands of many a meddling UX designer over the years. The best tools I ever used personally were Perl CGI scripts with no styling at all. Just fast page loads and buttons that do what I need.
A UI revamp is like coming home to a crime scene, or at least a messy kitchen.