Won't work.
They can't post your letters through a letter box, they can't open doors and slip them in internal apartment letter boxes, they can't do 'signed for', they won't cope with Japan's external street furniture, cables, PA networks playing the 5pm chime every evening (google it), and if they fly low enough they will eventually do 'helicopter rotor blade' injuries to cars, people, dogs, windows etc.
They will be useful for delivering to remote locations that can set up suitable landing sites (not easy, as they will need protection from the weather, but access for a drone) and for emergency supplies after quakes.
They will be useless at noting the ill health or distress of elderly people on postal rounds.
People who think everyone is going to switch over to the 'new normal', pivot to digital or do everything by app (or that everyone wants to) are wrong and should be sacked before they embarrass your company and mess up your bottom line. Any change will be to hybrid usage and will cost more in the short term, not less.