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".
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.