Best solution would be to copy AA and have additional Choice Benefits at thresholds beyond DM. Problem is, that objectively costs DL money so they’d need to weigh that against the additional revenue from those members who would attain those thresholds via credit card spending (they wouldn’t otherwise put on their DL cards) or via flying that would otherwise go to other airlines.
I have a number of colleagues in Delta hubs who have been conditioned for years to whip out their DL-branded Amex to pay for anything, the rationale being obtaining miles for the next family vacation. They don't chase status per se, and are not too concerned about jumping through hoops with different credit cards and rearranging their finances to maximize credit card benefits. Whether that changes in the coming years, I guess we will see.