Agreed; changing business software for change's sake is plain evil and will cost you customers. People still use the Bugzilla UI with 76 mandatory input fields and like it that way.
I believe that you're expemplifying survivor bias. People use Bugzilla despite its horrible UI (and in fact, I know of several teams that have GreaseMonkey scripts that create an alternative UI on top of the base Bugzilla one). Bugzilla does a lot of things right, but its UI is not one of them.
GitLab had similar problems with its UI, and they've been improving it. Personally I welcome the improvements (though we use CE and not EE internally).
Salesforce is a great example. It is amazingly "old fashioned", tiny and uses non of the UX paradigms of the last 10 years. However, it gets work done really, really well.
I'm disregarding the Lightning-UI versions for now.
I think Salesforce's advantage is its ecosystem and integrations.
However, I would not cite Salesforce as a reference for good UX. It is very labor intensive to get where you need. It takes a ton clicks to get places within the CRM—simple places like to find a contact profile. The Lightening UI makes the click areas for buttons and fields larger, but does not reduce the time or effort it takes to get places within the CRM.
My previous company, Trustfuel, built a business off of Salesforce being a pain for customer success managers to use. Note taking also sucks in Salesforce—Get Pattern's business is building a better note taking tool for AE's.
GitLab had similar problems with its UI, and they've been improving it. Personally I welcome the improvements (though we use CE and not EE internally).