Your math is wrong!
You're only making 4.15% on the 3% cash back, not the original purchase price. If you buy a $1,200 phone, you should get $36 cash back. If you deposit that money at 4.15%, that nets you an additional $1.49/year. 7.15% would be $85.80.
You really do have a reading comprehension problem! I never said I get 4.15% on the price of the purchase. I said GS has a savings account where you can get 4.15% on the cash back. Sheesh!
Let me do the real math for you:
Say you use the Apple card to buy an Apple product that will be $1200. Now you pay for it with the Apple time payment plan that is 24 months at 0% interest. 1200 / 24 = $50 monthly payment 3% x $50 = $1.50 you deposit in that GS account rather than take the cash to spend. At the end of the month you earn 4.15% / 12 x $1.50 = .00519 interest for that month. Your cash back now has a balance for the second month of $1.505 ( using rounding). Note: the actual payment will be higher due to taxes that do not qualify for the 3% cash back.
You don't collect the 3% of the full purchase price up front on a time payment plan. Only when you pay for the product up front. The beauty of the monthly compounding formula is that over the term of the loan the rule for compound interest or earning interest on interest actually results in a higher rate than the base 4.15%. Interest is always computed on an annual % rate so monthly you need to divide the % by 12.
Only each person can decide the options of whether to pay cash, pay with Apple card and pay the bill on the billing cycle for 3% on the purchase price or put the 3% on the monthly payment into a compounding monthly savings plan.
Usually I would take the cash back out each month because I have a way to get monthly compounding at 7% plus stock growth rate of 16% using a DRIP plan. But that is just me. I rarely pay cash for anything and work the cash back schemes and 0% interest wherever they are offered. It adds up.
I only use the Apple Card when I get 3% cash back while other cash back cards offer 2% except store cards like Amazon Prime that get 5%. I never carry a balance unless it is at 0% like Apple purchases. It's quite the deal Apple made with GS and I understand GS wants out but are bound by contract. When that contract is up we'll see what is offered then.