I don't mean to troll but I really don't get it. Why would language designers allow private methods instead of some naming convention (see __ in Python) ?
I searched for the answer and usual arguments are:
a) To make the implementation cleaner/avoid long vertical list of methods in IDE autocompletion
b) To announce to the world which methods are public interface and which may change and are just for implementation purpose
c) Readability

Ok so now, all of those could be achieved by naming all private methods with __ prefix or by "private" keyword which doesn't have any implications other than be information for IDE (don't put those in autocompletion) and other programers (don't use it unless you really must). Hell, one could even require unsafe-like keyword to access private methods to really discourage this.
I am asking this because I work with some c# code and I keep changing private methods to public for test purposes as many in-between private methods (like string generators for xml serialization) are very useful for debugging purposes (like writing some part of string to log file etc.).
So my question is:
Is there anything which is achieved by access restriction but couldn't be achieved by naming conventions without restricting the access ?