It’s funny you mention the postal service. If they defunded the postal service it would impact the government employees retirement plan. In short, because of the way they force the post office to account for their pensions it keeps the rest of the system funded. You cut postal costs, you reduce contributions to the Postal Pensions. You cut their pension contributions, the entire pension structure for all non military retirees goes bust real fast.
The entire federal budget is a mess and it is so Byzantine that a cut here or there would start a cascade over on the other side.
And it only gets worse as the boomers retire.
The boomers are departing this realm at 10,000 per day, and the rate is accelerating. Their retirement will not have as much impact as you might believe.
Dead people do not usually collect pension checks and have little effect on the Federal budget. They do seem to reliably vote Democrat.