With private fields, there's a clear signal that you will be crossing a line by doing all those shenanigans -- like a 'no running' sign at the pool, you violate

With private fields, there's a clear signal that you will be crossing a line by doing all those shenanigans -- like a 'no running' sign at the pool, you violate the rule and you accept responsibility for what happens next. With only public fields, there is no way to signal a rule. The pool no running sign is taken down. And, there are valid reasons for a library writer to want to warn people off using certain fields, but still have them in-structure. Work-in-progress, internal machinery that the writer may wish to refactor at their leisure, whatever. You could write that warning up in the documentation, but it's probably better as a structural affordance in the code, because then it's not just enforceable, but also lends itself to automation.