It’s not a $5 per line charge. T-Mobile is offering a discount of $5 per line if you use a debit card or bank account (eCheck) and use Auto Pay to pay your monthly statement. This is a discount T-Mobil is offering. It has nothing to do with Mastercard or any other specific brand of credit card.
Businesses change their pricing and discount structure all the time. I agree it’s disappointing when a discount changes and we have to pay more, but it’s a business decision. Fortunately, T-Mobile has competitors and you have choices in the marketplace.
AT&T does the same thing in offering their auto pay, except it’s $10 a line. I setup auto pay and then manually pay a day or two early with my Apple Card MasterCard. I still get the $10 a month discount for having auto pay setup and get 2% for using my Apple Card with Apple Pay. I don’t know if the same would work with T-Mobile, but it might work.
Apple and Goldman Sachs have nothing to do with discounts offered by T-Mobile or any other company. I’m sure they’d sympathize with the arrangement, but you do still receive 3% by using your Apple Card. You just can’t get an additional discount on top of the 3%.