As I recall, Delta made a deal with P&W to be an MRO for the P&W geared turbofan engines on the A220s and A320 NEOs, but couldn't get a similar deal with CFM for the engines used on the B-737Max. As such, it would seem Delta is highly likely to buy A320NEO aircraft instead of B-737Max for the remainder of this decade unless CFM changes their mind about engine maintenance contracts. It seems to me this shouldn't be so much a Boeing vs. Airbus discussion, as a CFM vs P&W discussion. Since P&W engines are only pared with the A220 and A320 families and not with the B-737Max, that seems to limit what Delta is likely to buy for now.
Also, in their Apr 22, 2021 press release, Delta said "Delta’s total purchase commitment for the A321neo will now be 125 firm aircraft, with 100 purchase rights." As such, with 100 existing options for the A321, they definitely have room to grow their A320 family order in the coming years if they want/need to add some incremental aircraft. Also, I don't see Delta wanting to add a new fleet type (i.e. B-737Max) unless they have a need for something like 75-100 new planes AND they can get CFM to make a deal. Sure, one group of pilots could be used with both the B-737Max and B-737NG, but there would still be extra incremental/difference training. Plus spare parts to buy, and the like. They'd be better off just converting some of their 100 options on A321s rather than adding a new sub-fleet.
Also, in their Apr 22, 2021 press release, Delta said (emphasis mine) "The new aircraft [A321neos] will be deployed primarily across Delta’s extensive domestic network." I don't think Delta is worried about planes with A321XLR type range at the moment. I think they are planning for planes with enough range to fly ATL-SEA, but that are also good (and cheap/light) enough to fly routes like ATL-MCO. Delta doesn't want to pay extra for heavier planes that have excess range compared to their needs.
Also, for those thinking Delta might care about only buying modern, fly-by-wire aircraft, keep in mind we are discussing the company that just last year retired their MD88 and MD90 fleets. And they still plan to operate MD95s (B-717s) until roughly 2025. Delta cares about high dispatch reliability, minimizing debt, maximizing their credit rating, cost of operation, being able to buy replacement parts, and the like. I really don't think they care if their fleet is "modern."
Realistically, I would GUESS this is what Delta is telling their suppliers:
Boeing: What are your plans for a 150-200 seat Yellowstone-1/NSA/737 replacement for delivery starting in about 2029-2031, give or take? We'll be in the market for some planes of that size then; either some NSAs or heavily discounted, last off the line, B-737Maxes.
Airbus: Got any plans to not only launch the A220-500 (at a price and with a seat count and range that is to our liking), while also funding and building a final assembly line for those planes in the USA so we can avoid import duties? If so, let's talk about some possible orders with deliveries starting in either the 2025 or 2030 time frame (assuming you also are going to do a nice efficiency upgrade on the A220 program by 2030).
Airbus and Boeing: Got any plans to launch and start delivery, in 2025 or so, of something with more seats and a longer range than an A321XLR but also with fewer seats, a shorter range, and a lower per-seat cost (for both in terms of initial acquisition cost and long term operational cost) than a B-787-8 or an A330-800? We're expecting to run out of B-767-300ERs around then and might want to keep flying routes of similar distance and with similar passenger demand. Or we might just give up on operating those routes and just hope our customers will agree to fly to partner hubs in Europe, Asia, and S. America on larger aircraft and accept an extra connection. Or maybe international flying won't have recovered by then and we will just shrink. So really, we're saying please try to impress us into buying new aircraft of this size and range or we're just going to shrink our fleet. (And maybe just upgauge some key hub to hub routes to A350-900/1000s.)
And given that the responses to all 3 of the above questions is likely be along the lines of "We have no such plans."
MROs and parts suppliers: We've got fleets of A319/320 CEOs, B-737NGs, and B-757s that we intend to keep flying until at least 2030 and maybe longer, so you can count on our business up to, and perhaps beyond, 2030, so please don't be closing up shop before then, because you'll have a loyal customer in Delta. Well, unless you increase your prices, because if you do that, we're going to just immediately exercise our option to buy 100 new A320s and cancel our contracts with you. But otherwise, it's nice doing business with you.