Languages are not static, dictionaries are not made of stone. A word's primary usage can definitely change. Many words are like this: "[to] retard" is one such example where the original definition has been superseded by another definition in general usage. One can walk around with a dictionary and a prescriptivist mindset and tell people their language is "wrong", or one can acknowledge that definitions and usages change over time.

The only time I've ever encountered this usage of public intercourse is in older texts. Certainly no one my age would use that instead of "public discourse".

And that has nothing to do with levels of education or "childishness". One can be highly educated and have a diverse vocabulary while acknowledging the fact that certain usages are more popular than other usages, and they can change over time. Languages are illogical and beautiful in that way.