Those days have long passed. People have communications now essentially no matter where they live, post office or not post office. In those days it was the only means of long distance communications and was essential - communications is still essential, but the post office no longer provides it.
Methinks USPS should revert to the library model: office in every town, with office-to-office delivery guaranteed as a Constitutional obligation. The door-to-door thing is a step beyond what’s required, and is a relatively recent phenomenon.
Just to stir the pot: one could argue that due to technological advances the Constitutional “post office” clause could cover email as well, requiring USPS provide at least basic email service.