The CashPlus "checking account" is technically a brokerage account that just uses the Bank Deposit Program (BDP) for settlement funds. That account alone qualifies you to hold the Morgan Stanley co-branded Amex Platinum card. If you close all of your Morgan Stanley accounts eventually that will trigger closure of the Platinum card.

Say you go the "hands-off" approach and keep $700 in the CashPlus account to make sure there is always enough to pay the monthly fees over the course of the year. That $700 held in a settlement fund like FZFXX at Fidelity could earn $31.50 in interest, meanwhile the BDP program would pay out less than $0.07 on that $700. The difference in interest rate alone erodes most of the $35 advantage ($660 vs $695) right there.

If you factor in the $175 AU card, that is the is the situation that will always win looking at the math on a spreadsheet. What could complicate things is the effort to maintain the qualifying Morgan Stanley account. It becomes an account that you need to monitor for fraud and you need to check the statements and pay attention to the quarterly FYI inserts for any rule changes that pertain to the program.

Staying on top of the Morgan Stanley program and rules costs you something in terms of time and effort, though it's difficult to assign that a dollar value because you're not paying an explicitly defined fee.