A good mirrorless setup with a sharp prime lens should last you five to ten years. You want gear that handles dusty streets, low light, and packed bags without breaking, so it pays to buy right the first time instead of upgrading next year.
Camera brands drop new bodies every spring, which means official factory restocks of last year's model hit the market at deep discounts around May. If you grab a certified refurbished camera then, you get the exact same sensor and build quality for hundreds less.
That 15-month intro window is a strict payoff deadline, not free money. If you still have a balance on your Discover it® Cash Back card when those 15 months end, standard interest kicks in on whatever is left, which quickly wipes out your savings.