Taxes being voluntary doesn't starve the government. In fact, the IRS isn't even going to care if most people just stop paying taxes with USD. They only target people who are worth targeting, and that doesn't change with Bitcoin. It just gets harder.
Yup, they'll actually have to offer services that people will
want to pay for.
Sound familiar?
No, they'll still collect taxes from people that want to pay them. There are a lot of those people.