Most mail needs to go into a "we'll deliver it when most cost effective; max of 14 days".
I really like this idea. Seems like they'd be spending resources where they are really needed. Rural places like where I live would get less frequent deliveries than cities and I'm okay with that. Like you mentioned, there's First Class if you really need something delivered soon-ish. To some degree, this system is already appearing with the end of Saturday deliveries.
Bigby for president!