I think that it might be possible to contest a clawback under the present circumstances given the degree to which the card's benefits are changing (e.g. the DL Plat losing paid lounge access and both losing Basic Economy lounge access as of early next year), especially if you got the card close to the massive wave of changes - say, on that last promotion before the devaluation. This has come up elsewhere, but I'd argue that in that narrow set of circumstances, where Amex was pretty obviously not clueless as to the impending changes but kept hawking cards regardless, bait and switch probably applies. This is likely a very narrow subset of people, FWIW (probably a few thousand at most).

That being said, I have to wonder - if someone already spent the DL miles, flew the flights, and had no plans to fly DL again, would it be possible for them to just leave the account balance negative and walk away?

 

That's relevant for AmEx Platinum holders who didn't have to earn status on delta and worry about Main Cabin tickets