I flew 1MM on DL, but I live in DEN. So I switched to UA about three years ago. UA is certainly convenient, but DL is a MUCH better airline. that being said, DL has zero reasons to fly beyond the 125k miles (and 15k).
The changes with UA are really silly. They have a similar partner scheme to DL, so most UA fliers are now busy booking INTL flights on partner airlines and crediting them to UA. The 54 flight requirement is really pretty easy and 18 k is not hard to reach with three premium economy trips on any *A partner.
With all the changes I might actually be able to qualify on AA too. I just picked UA first since I already have platinum from last year and I can pretty easily get 1k this year. AA offered to status match me though based on my diamond status with DL. I know, first world problems for sure but if I have to spend a large portion of my life in a pressurized metal tube at 35,000 feet I would like to maximize the rewards.