The old wisdom "mark it private unless you have a good reason not to" made sense in days when it was written, before open source dominated the developer library space and VCS/dependency mgmt. became hyper collaborative thanks to Github, Maven, etc. Back then there was also money to be made by constraining the way(s) in which a library could be utilized. I spent probably the first 8 or 9 years of my career strictly adhering to this "best practice". Today, I believe it to be bad advice. Sometimes there's a reasonable argument to mark a method private, or a class final but it's exceedingly rare, and even then it's probably not improving anything.

When mocking is deemed profitable:

Programmers should be encouraged to understand what is correct, why it is correct, and then propagate.

The more important point comes from a program design perspective. Here, "programming to an interface" means focusing your design on what the code is doing, not how it does it. This is a vital distinction that pushes your design towards correctness and flexibility.

But, let’s be pragmatic for a second, the 80/20 rule states that you get 80% of the value from 20% of the work and a couple of simple tests are vastly better than no tests at all. Start small and simple, make sure you get something out of the testing first, then expand the test suite and add more complexity until you feel that you’ve reached the point where more effort spent on realism will not give enough return on your invested time.