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.