What you're talking about is UNIVERSAL SERVICE as required by law. It's backed by 50 US Senators ~ not likely that's going to change any time soon.
First Class mail cannot, at any price, justify the current Postal Establishment. You'd get down to weekly delivery. There are a whole lot of US Congresscritters who'd be PO'd like you wouldn't believe.
The USPS is simply the latest manifestation of the demiurge located in the heart and bowels of every politician to COMMUNICATE WITH THE PEEPS.
We can only do that which is legally and politically possible. That's why eliminating Door Delivery is possible ~ the people getting it today are mostly "THE RICH" and those who live in slums!
Neither group is very popular so it's time to go after the good stuff they get that is denied to others.
I see Kiosks in the future of both the richest and poorest Americans~!
“You’d get down to weekly delivery”
Which is exactly what I propose.