Why is the coupon rate of existing debt irrelevant for finding the cost of debt capital?

Question:

Why is the rate of existing debt irrelevant for finding the capital?
Cost Of Debt
The cost of debt is the effective interest rate a company pays on its debts. It’s the cost of debt, such as bonds and loans, among others. The cost of debt often refers to before-tax cost of debt, which is the company's cost of debt before taking... Coupon
A coupon or coupon payment is the annual interest rate paid on a bond, expressed as a percentage of the face value and paid from issue date until maturity. Coupons are usually referred to in terms of the coupon rate (the sum of coupons paid in a...