While you are certainly entitled to your opinion and I respect it, this is rather subjective depending on a lot of factors.

I've been elite on all of the US3, but chose AA because I couldn't take UA anymore and because of AA's superior network. Their hubs, routing, frequency of flights, airport lounges are just the best of the legacies for me.

I would go so far as to say Delta is the best US airline, but their system of routing everything through Atlanta creates a weak point and made them prone to delays and cancellations due to weather in ATL, etc. It just wasn't for me. Pricing was also a lot higher to Asia.

It was probably down to UA or AA for B6 to partner with. I would have thought UA would have been the better fit, but perhaps they weren't interested or AA offered the better deal.

In the meantime, I am hoping for AA codeshares on B6 with full miles redemptions and award inventory. One can dream!