I think Amazon's $750/mo should be thrown out because its trivial to get 5% from the store card or any of the other numerous ways.
I'd rather not use a hard pull and a new account for a card with a $50 signup bonus just so I can keep getting 5% at Amazon. Also I get Prime shipping benefits for free because a friend added me as a guest on their account, and per the T&C you need a paid Amazon Prime account to get 5% back from their store card so that essentially adds an annual fee.
As for the "other numerous ways", there are caveats. Sure, you could buy Amazon gift cards at a grocery store, but:
your Amazon purchases wouldn't be eligible for card benefits (extended warranty, price protection, etc.)
whatever card you use to buy the gift cards probably has a limit on category bonuses that is lower than the Sallie Mae MC limit on Amazon purchases ($9000 a year) and which you'd have to share with other purchases or MS
you'd need to plan in advance (i.e. always have money in your Amazon gift balance)
The gas also could be thrown out since like I said better to get a Costco Cash card at Freedom 5% and also get the cheaper Costco gas prices
But then you'd need to stock up on cash cards for the entire year during the relevant bonus quarter, and you wouldn't be able to use that quarter bonus for other things (either legitimate spend or MS).
Not to mention this assumes you will always get your gas at Costco which in my experience is rather hard to do (e.g. don't plan well and get low on gas far away from Costco, or during a roadtrip, or if you fly somewhere and get a rental car).
*tl;dr: *yes, you can get similar benefits with other cards, but you might have to open new cards that don't necessarily have a worthy signup bonus, in most cases you'd have slightly lower rewards and lower limits, and you'd have to jump through all kind of hoops to get there. The Sallie Mae MC was great in its simplicity: you didn't have to think about which cards should be in your wallet every quarter, you didn't have to plan Amazon purchases in advance or delay them, you didn't have to worry about where you should be stopping for gas, etc.