Have to agree with others who have said that if your only metric is the Skymiles program and benefits, then sure, it's been "frustrating". But that's simple economics. The economy has improved. The market has consolidated. Supply and demand says DL has to do far less incentivizing to fill seats. Sucks for those of us who enjoyed the perks for so long but that's (again) supply and demand.
But overall, I've noticed improvement in areas that matter more as average passenger. I've found DL's operational reliability has improved significantly. Whether that's actually true across its operation, I don't know, but that's my experience. I find their IROPS recovery better than other carriers (Yes, it probably helps that I have status with DL). I also still find DL's front-line employees to be among the best. I also find DL service recovery (customer complaints/feedback) to be among the best at "making it right" (again, it probably helps that I have status here). I've found the onboard product mostly improved over the past decade. More routes now come with meals and better snacks in F. C+ has seen improvements. SkyClubs have gotten way better in the last few years (I admit I was really spoiled being in Seattle for a few years with the Flagship Club there and it sucks now being in SAT with no SkyClub
).
My top alternate airline when I can't go with DL is AA, and I haven't had the problems that many others on here chide them for, though flying without status can feel rough. My biggest frustration with AA is domestic IFE, or lack thereof, compared to DL. I do like that DL has way more aircraft equipped with seatback screens while AA only goes with AA Studio or whatever they call it throughout most of their fleet. I don't get the same "feel" from the employees on AA. It's not that I have had bad experiences on AA. But with AA, I get the impression that it's a job and nothing more or less. On DL, to most front-line employees (and I'm guessing behind the scenes too), it's more than a job. DL is also a culture they're proud to be a part of. That's ultimately what for now, keeps me loyal to DL, even when it's been tempting to switch to AA, especially now living in Texas where AA would be much easier.