It is going to be HUGE and do incredible damage. Retirees will be hurt catastrophically, costs of government borrowing will skyrocket, programs will be cut massively, workers will abandon government.
The issue is that the costs of meeting the prior obligations will be completely impossible. States, cities, and school districts will go bankrupt, massive numbers of people will be fired, and taxes will go up while service goes down.
Unfortunately the politics prevent a fix today and prevented a fix 10 or 20 years ago when it would have been much cheaper. The fix will happen in 10 to 20 years and it's going to be horrific.
As for Roth IRA. I don't trust the government to honor their end of the deal. I can see them taxing withdrawals for the top 10% in the future.
It's not just former employees that will suffer. Everyone suffers when states have to pay higher interest rates on their bonds because of poor credit ratings (that's what bankruptcy does to a state) or have to pay more for workers and contractors. Everyone will have to pay higher taxes and everyone will receive less from the government.
I cannot understand the logic of not making good on government obligations. You can take the position that the government should stop promising pensions, but how can anyone think it is a good idea for the government to fail to pay for the pensions it already promised people (or, frankly, any other promise the government made)?