First, B6 has the same boarding cut off as the three legacies at T-15. The purpose is to permit the GA, the Captain and dispatch to close the flight, compute weight & balance and allow an ontime push. If you arrive at T-14, you are late and if B6 (or UA, AA or DL denies boarding you are SOL).

Second, if all ticketed pax are onboard, there is no reason to hold the flight, whether it is T-16 or even earlier. What is B6 waiting for. Sure, it was nice back when DL or Eastern before it ran the Shuttle as a continuous service. But, those days are long gone. B6 does not market a Shuttle and the AAS/DLS are mere branding gimmicks.

I see no reason not to close the flight at T-14 at the latest and to lose it earlier if all ticketed passengers are onboard. The problem lies in people who cut it too close and think that they can get away with it.