A lot of users do care for fit and finish, but stick around with crappy UI because it’s not worth rage-quitting over.
They do move when better UI + same-ish functionality comes along.
The one sort-of exception to this is Enterprise Sales, where the people buying the software don’t always use it. But even there, corporate purchasers do get flak in annual reviews / feedback cycles for especially crappy enterprise software — so even there, especially crappy UI will catch up with you.
But the question is will they move from a Java UI to a native windows UI, all else being equal. What if the Java UI is better ? What if the Java application is better
I think you are correct, but I think your example is a bad one. Most people do not have a realistic (their employer will use it at work, it even occurs to them that they could use something else) alternative to MS Office so will use it whatever MS does.
The sometime subjective aesthetic reasoning doesn’t mean the application can’t succeed. office ribbon is an example