There's enough details out there and "shop talk" on other places on the internet that lend this to be fairly credible with enough detail that is more than just typical pilot or FA rumor mills.
Devils advocate here, but Boeing is the leaseholder, likely gave DL a great deal on the lease extension on the 717. After all it was an orphan fleet type that no one else would want to digest all 91 frames. Secondly, if DL walked away they'd go and order more A220s from Airbus. Thus it was in Boeing's interest to make the 717 financially viable at DL longer since it keeps the replacement off of a competitors orderbook for that much longer.
There are a lot of sub-800 mile routes out of ATL, DTW, & MSP where the 717 makes a ton of sense and the fuel efficiency of newer aircraft is not fully realized and/or offsets the much higher acquisition costs. Basically all the 'ole DC-9 flying of yesteryear.
Another point, is that maybe that this hedges against some of the teething issues and production ramp-up issues of the A220. The 717 is a known commodity, its proven technology at this point.
First, all good and valid points.
I would add, if the lease is as cheap as I expect:
1. Allows DL to use the fleet as a power by the hour asset. In other words, only fly the 717 when passengers pay more.
2. Cheap insurance for growth. If the market demands growth, fly these aircraft more.
3. Cheap insurance for a downturn. If a recession hits, better to park low lease payment aircraft or RJs.
A great hedge play. I personally wanted to see A220 orders as that hedges fuel, but not a recession.
IM messages to mods on warnings and bans will be ignored and nasty ones will result in a ban.