On DL, there's no MQD (spend) requirement if you can put $25,000 on a DL AmEx card annually. Doing this with the DL AmEx Plat credit card also gets you 10,000 MQMs toward status (and often more as a signup bonus, which isn't wasted due to rollover if you reach at least FO). Moreover DL miles don't expire (until you do), so it's hard to see a downside of travel and crediting to DL if you don't pay a lot more to fly on DL and partners.