Did you ever actually refactor code with a significant test suite written under heavy mocking?

The mocking assumptions generally end up re-creating the behavior creating the ossification. Lots of tests simply mock 3 systems to test that the method calls the 3 mocked systems with the proper API -- in effect testing nothing, while baking in lower level assumptions into tests for people refactoring what actually matters.

You might personally be a wizard at designing code to be beautifully mocked, but I've come across a lot of it and most has a higher cost (in hampering refactoring, reducing readability) than benefit.