That's not how business continuity works. You don't just pull the plug on your vendor when its a critical component of your business (or in this case, a public service).

What are you going to do if you have a dispute with your payment processor? Just stop processing payments while you switch providers? Its like no one here has run a legitimate business before.

I think a dispute with a payment processor is an issue on a little different scale than the leak of every American's credit history and PII.

Regardless of what the options are, it's a colossal PR misstep that deserved a much more thorough explanation from the IRS than "they're the only ones who can provide this service that we can't readily explain".