Keeping people employed just because is probably the reason the USPS is having the issues it already has. Cutting the workforce and cutting every other day of delivery could make a HUGE impact to their bottom line. Likely the same mail trucks could carry and deliver two days of mail every other day without needing to put more trucks on the road.
No, the USPS is having to fully fund pensions for people not born yet is what's causing the problems. If you look at the profitability graph it nosedives around year 2000 or so purely because Bush Jr and the GOP were trying to kill it by forcing it to fully fund pensions for the next 75 years or so, which includes funding pensions for people not born nor employed by the USPS.
Most companies aren't doing this which means if they go under, there goes all the pension funds. USPS pensions being fully funded means those people keep their funds when USPS goes under.
It's basically been a way to kill the USPS without killing the USPS directly.
Before this ruling came out, the USPS was really quite profitable, and those profits could've been used to fund the pensions until the obligation was met rather than force them to pay for pensions fully by going into debt.