"The A220 will allow Delta to upgauge more 76-seat regional jet flying to mainline aircraft, a move it began with the introduction of the 110-seat Boeing 717-200 in 2013, and likely replace some of the 717s down the road. "
While the above is true, Delta only got the 717s when AirTran was bought out by Southwest and they needed to divest the 717s to maintain a 737 fleet only. It all turned out rather well for Delta, but the corporate vision to move to mainline jets was facilitated as much by Southwest/AirTran as Delta.