I’ll answer from (sort of) the opposite side of the coin. I’m a DL elite who also has status with AA via a challenge completed two years ago (and extended due to COVID and will be completing one of the promos to extend it again this year) and received UA Silver through my Marriott Ambassador status. It’s nice having status across all three for when I don’t have the options (USG traveler stuck complying with GSA city pairs at times) but while I’ve contemplated switching my primary carrier for my personal travel and trying to build status on AA, I’ve never pulled the trigger.

I just find DL’s service and operational reliability superior to the other two. And while it wasn’t popular with many FTers I was pleased with how DL handled COVID early on (seat blocking especially) which has kept me with DL. Even when I was in AA and UA territory living in SAT, which had ample flights to their mega hubs at DFW and IAH, I almost always opted for DL when I had the choice. The two exceptions were a trip to Hawaii in PE there and J back on AA, which from TX, AA is the better option unless you want to back track to ATL (which also tends to be way more expensive and no PE option on DL) and a one-way on UA when DL’s fare was several hundred more.

(I will be cheAAting on DL over the next couple months when I take a couple trips to complete the promo to retain my AA status but even then I have DL segments with those trips - only taking AA for what I need to).