Its true. We pitched this exact thing. You want IE6 support, we need 30% more time on UI work at least. They either agree or take away IE6 support. And I don't make this crap up.
YES. This is exactly the right approach. It DOES take more work to support. If they want to pay for it, fine. But it also exposes the internal "upgrading is expensive" excuse as fallacious. NOT upgrading is also expensive. At some point the balance tips and it's time to move on.