I'm pretty sure most people who fly enough to really benefit from PreCheck would gladly fly their preferred legacy of choice with their first class upgrade and

I'm pretty sure most people who fly enough to really benefit from PreCheck would gladly fly their preferred legacy of choice with their first class upgrade and elite service desk standing by in case of trouble... B6/VX/WN are certainly fine establishments, but they hardly cater to the same market of flyer that the trusted traveler programs target.