I have 1k and diamond on DL. Honestly, the ability to pick and choose is pretty nice. Sometimes UA is better, sometimes DL. The last time I flew through EWR I connected to the EWR-DEL flight. every single flight on that itinerary was delayed. I even got kicked off a flight because the 1K desk "helped" me and rebooked me later. Instead of stewing about nonsense like that, I just book DL the next time. When DL inevitably screws up I go back. I fly 50k a year out of DEN. That probably wouldn't get me GS but it might get me 360. If I could get guaranteed invite status I might concentrate my flying.

Absent that, no way would I concentrate any business on one vendor. The risk is too great.