Absolutely not. If you don't pass the IGA, you're then subject to FATCA itself. FATCA itself is enormously complex and enormously expensive, and the implications of non-compliance would be astronomical.
Let me throw a couple of numbers at you. Canadian foreign direct investment in the U.S. is right now, total stock, $318 billion, and U.S. source income flowing back to Canada from the U.S. is $42 billion every year. Imagine subjecting that to a 30% withholding tax.
Imagine subjecting the holdings to a 30% withholding tax on the gross proceeds of sales. I could sell something, lose money, and still get taxed on the proceeds of sale.