"...after which Delta will have the option to purchase the aircraft at the then-prevailing market value."
And that turns out to be a very-valuable lease term for DL. The prevailing market value for these frames isn't high. So DL gets a very good deal if it wants it. As Lahey used to point out, he can make up for a lot of inefficiency in a new-build frame over time just by lowering the price. The kind of prices these frames can be had for is an important component of how economical they will be to continue to operate.
In high utilization service the purchase price is about 15% if the cost, fuel (today) about 30% and maintenance about 15%.
Buying new would save about 30% in fuel (recall, the A220 cruises 4,000 ft higher, and subsystems that save fuel. The weight is the engines that save fuel. Maintenance is about 20% less.). So buying new saves about 13%+.
However, in low utilization duty, it would make sense to keep the 717s around. By low utilization, I mean the part of the DL fleet that doesn't fly much on Tuesdays and Wednesdays and is parked in low season except for the busier days.
I see a need at DL for some low utilization aircraft. The question is, when do Douglas parts become so dear it isn't economical to support the type? It typically takes 400+ aircraft in service to create enough economy of scale. If anything, economy of scale is climbing with animation. (Save 13% everytime production doubles instead of the old 10%.)
IM messages to mods on warnings and bans will be ignored and nasty ones will result in a ban.