DL offers 1" less in normal seats and 2" more in C+, which FOs often, but not always, get. Delta also has many planes with wider Main Cabin seats than AS and WN's 737s. C+ also has free alcohol.
You don't get upgrades or C+ on DL unless you pay "get me out the hellhole of Basic Economy" tax, nor can you change or cancel a ticket. All of these combined be a major dealbreaker for me. On my routes, AS and WN give me full benefits on lowest fares (and generally speaking on my typical routes there's fare matching between all three airlines, or AS/DL). DL does not. DL thus wouldn't be an option I often consider (I might do it occasionally if as the low cost leader it makes sense... just like I have flown NK occasionally). But I tend to avoid airlines that punish me for low fares, except on a transactional basis where I can discern a clear benefit (which I guess is what they want).
I also don't mind WN boarding.
Oh, yeah. If the fare goes down on WN (or AS), I get the fare difference back in future travel. (I fully realize this would not be a feature to someone who hates WN- more WN flights.) It's not uncommon for this to happen to me; I probably get about $100-200 in fare rebates a year.
I just don't see how it's better than AS as an MVP or DL as an FO.
Note that I said I haven't defected from AS (though I fly a decent amount of WN), but if I did... how about "I don't have to write DL checks for $3000 to qualify for Silver on MQDs, or funnel $25,000 in purchases through a particular AMEX card, or pay get me out of Basic Economy tax"?
It's obvious DL works for you as a Platinum. There's a lot of folks it does work for. It might even work for people in this thread. I don't think DL works for me as a cost-conscious marginal/lowish elite who would struggle to meet the MQD threshold, is not interested in the DL AMEX and flowing through $25k in spend to meet MQD waivers, and often would be stuck in Basic Economy on low fares anyways, and thus not getting those C+/F upgrades.
If I fell out of elite status with AS (or AS joins the "spend more money or it will suck for you" crowd), I am unlikely to gravitate to an airline with Basic Economy punitive experiences in coach, or try for status on an airline that has spending qualifications for status. This eliminates DL/AA/UA. That leaves me with... drumroll...WN. (I'd probably go with WN for most flights, AS/DL/what have you on a case by case basis).
Does this make my reasoning clear?