There is the question of whether, in 2017 once this has been sorted out, what (if anything) they'll change to remain profitable with the increased employee costs going forward but they were quite profitable before this happened so I don't see too much of an issue.
Although the federal government DOES HAVE special economic status so I'm not sure how you think that's particularly weak unless you think reality is pretty weak.
$100 in revenue - $75 in costs - $50 in off balance sheet debt is not profitable.
The special economic status of the federal government simply means that the taxpayer is on the hook for the USPS's hidden debts. No one disputes this. What's under dispute is whether the USPS should be allowed to incur hidden debts on behalf of the federal government (PAYGO accounting), or whether their debt should be transparently included in the federal debt (ERISA accounting).
USPS is NOT put in a worse situation than private companies. It had a privilege that put it in a much, much better situation revoked.
The fact that future governments can (probably) be relied on to make good on promises made today is a "weak argument" for just kicking the can down the road instead of funding the liability immediately.
In 2017, USPS's costs will go down, significantly, because they no longer have the backlog of pensions to fund, they will just to funding pensions on an ongoing bases. Exactly like private companies. At that time, USPS will be in a situation where it can be reformed, because it no longer carries around a glut of unfunded liabilities that will be dumped in the government's lap if something goes belly up.