You've said that it can be seen as a long-term investment for society. Definitely! The United States has taken such long-term investments in the past as it subsidized people settling all over the continent. However, there's a difference between investing in something that has a future and throwing money after a lost cause. Some places are simply rural. Some people don't want increased commerce or activity around them. Plus, if you're talking about government revenue from taxes, if these communities withered away and the residents moved to less rural areas, the government would make the taxes off of them there - and potentially greater taxes since they'd be in a more economically active area.
That said, I think that a smart idea might be to create tiered service that is aware of the investment proposal. So, new communities get "first class" service for their first 15 years - mail delivered every day just like it would be in Boston. After the initial "investment period", service will be determined by the cost of delivery. Urban areas might get 6-day service. Suburban areas, 5 day service. Rural areas, MWF service. Highly rural areas, TF service. Mega-rural, no houses within 15 mile type places get mail once a week (considering that, in this scenario, a carrier would only be delivering mail to a maximum of 28 households in a day, I think that sounds somewhat reasonable). That would keep commerce running while reducing costs.
And I'm not sure there'd be too much objection to such a plan. I have a co-worker who is from a community that you can't get to by road - you have to go by boat. It is disconnected from things and that's how the residents like it to be. Granted, it's still nice to be able to get deliveries, but they're more into making things for themselves.
Does that sound like an interesting proposal? It's definitely important to keep people connected, but maybe they don't need to be connected every day. If one has situated themselves in a location apart from society, maybe it's ok that we stay connected with them, but not quite as connected as we do with those who have located themselves at the center of society.
EDIT: After talking with my co-worker, most rural places have the same every-day service, but there are middle of nowhere places that do only get mail once a week.