If Schwab was introducing a card from another issuer, that issuer would need to buy that portfolio from American Express. When Costco moved from American Express to VISA, Citi paid American Express $1 billion premium over the outstanding balances for the accounts. If there is no portfolio sale, American Express would convert to standard Platinum cards as they did when the Mercedes Benz Platinum card ended.
I have been with Schwab for a very long time. Schwab used to have a 2% cash back visa. At some point Schwab solid it off to Bank of America who immediately diluted the cash back percentage. I then closed the credit card account. With Schwab, with the $200 annual bonus, that makes on AU card free. As long as I and my wife travel, Amex Platinum is worth having. But when we stop traveling I will be back to using our Fidelity 2% cash back from Elon. It is a nice card too. Only downside there is a foreign currency transaction fee.