They don’t, Delta is replacing the MD’s with 737-900’s and A321’s. They have enough on order to replace the MD’s and grow significantly.

They are also replacing some of the 757-200's with them (and saving a few bucks an hour pay rates at the same time.)

The 757 retirements were done a year ago. The current 757 fleet is planned to remain static for several years.

Most of their routes wouldn't be profitable with the A220. Such as the evening flights from the east cost to Vegas. MCO or FLL for the vacation / cruises. Central or South America. There's a reason they put 180+ seats on a A320 or 220+ seats on a A321. They have no problem filling them.

looks like the routes announced are mostly current -900 routes flown by Endeavor and SkyWest as well as mainline with 717s and a319s

Lga-bos
lga-dfw
dtw-dfw
lga-iah
slc-dfw
slc-iah
msp-dfw
msp-iah
dtw-iah
jfk-dfw

Most of the NYC routes are YX routes, for what its worth.

Endeavor does the most flying for Delta out of LGA. More than Delta. But JFK-DFW and such are Endeavor as well. However, LGA-DFW has been YX lately as I've seen, but I wouldn't call the generalization as "most" in your post as an accurate assessment.