The best way to know the value of an airline is after you become a free agent.
I stopped looking exclusively at DL flights in 2023.

I now enjoy more flexible points from AS and AMEX. I realized SkyMiles are the lowest value FF "currency" out there.
My TPAC and TATL J experience is also considerably better avoiding DL there.

Still, becoming a free agent has also made me appreciate the SkyClubs compared to domestic alternatives, DL's check-in, consistency of domestic hard product, and baggage handling, and not take it for granted like I did as a loyalist.
DL still earns my money most of the time (largely domestic due to schedule) but trying other carriers did convince me that continuing to requalify for DM was still worth it without committing to unfailing loyalty.