Delta typically has the better in-flight product, though I agree that SkyMiles as a program is generally a waste of time.

That said, there's nothing stopping someone from crediting DL flights to, say, Flying Blue and using that program instead. I was able to snag FB Gold with Bilt last year. The lounge access is nice as is the ability to use those Bilt points on Alaska, Hyatt, or Flying Blue. Nowadays the miles from CC spend are easier/more valuable than the miles from actually flying.