Did you ever actually refactor code with a significant test suite written under heavy mocking?
I have. The assumptions you make in your code are there whether you test them or not. Better to make them explicit. This is why TDD can be useful as a design tool. Bad designs are incredibly annoying to test. :)
For example if you have to mock 3 other things every time you test a unit, it may be a good sign that you should reconsider your design not delete all your tests.
It sounds like your argument is “software that was designed to be testable is easy to test and refactor”.
I think a lot of the gripes in the thread are coming from folks who are in the situation where it’s too late to (practically) add that feature to the codebase.
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.