Worrisome, though, is "However, we under-appreciated that incremental revenue from buy-up would be more than offset by share loss to legacy carriers without similar Basic Economy offering."
It's not perfectly clear, but if that terms is used accurately, it means pax have been booking away to DL and AA--not B6, WN, and AS. If so, that means that AA's full-fledged jump in the pool may see less of "booking away," and UA may reconsider.
Then again, how could Kirby know that pax are booking away to legacy carriers w/o BE, and not to non-legacy carriers?