The US Postal Service is an American Institution which predates the Declaration of Independence. It was created and exists to bring our country together and if not for it our country would be a vastly different place today. It is the only federal agency which interacts regularly with the whole of the general public. It is the only agency who's employees serve you personally and may actually care who you are.
Does it have serious problems? Yes.
Does it need to adapt to future public needs? Yes.
Reducing the delivery week to five days may seem sensible. It may seem trivial in a modern world of instant communication. In the short term it may help the problem but it is a very bold step down a very slippery slope.
One day is gone, what comes next?
Would you miss it if it was gone?