Of note regarding money markets: Nothing beats the Vanguard money market. It has the highest return currently (floating at around 5.28 last I checked) combined with the lowest expense ratio of any ‘normal’ fund (one that doesn’t have a 100k+ minimum deposit required).
Schwab also has a higher return than fidelity, and a slightly lower expense ratio.
Fidelity has the worst returns, and the highest expense ratio. However, the difference between the three are mostly inconsequential unless you’re plopping a lot of money in there. Still, free money is free money, and all three brokers are reputable.
Vanguard has the worst customer service out of the three though, so that kinda negates the better returns, especially if you ever find yourself in a position where you need it.
Upside of fidelity are the zero cost index funds they offer.
To the OP’s point, I think you make a strong case. Still probably good to have a regular local credit union as backup and to easily deposit cash, (fidelity’s unusual bank account number can very occasionally not jive with some things reportedly).
Also, there is a security concern if your brokerage account is attached to your cash account, as I believe it would be possible to drain the brokerage account with the cash accounts debit card, as the money market assets in the broker account will be liquidated to cover expenses incurred in the cash account if it becomes emptied.
I also never looked into how Fidelity deals with fraud. Shwab’s cash account is a real bank, while fidelity’s is technically not, so there might be some differences there that I’m not aware of.