[maxerickson]

Because under a stronger definition of tainted, they cannot be trivially mixed, they pass on the contagion.

The US government can ostensibly stop accepting Zerocoins. In that event, so would any party that wanted to trade with them. There wouldn't be any need for a new blockchain, possessors of clean coins would simply have to decide how much extra value that part of ecosystem provided.

That last sentence mixes up with your point 3, tainting all the coins would be a collaboration between owners of clean coins and service providers, there is at least room for owners of clean coins to be careful with them.