It seems the existential threat to an airplane remains the same: cockpit access. Note that in some areas, taxis have more security than an airplane, in the sense that there's no way to escalate from passenger to pilot (dividing wall).

Even today, I see pilots open the cockpit during flight to use the toilet. Usually, the FA blocks the passageway with a cart. It seems plausible to me that there's a short period of time that a well-trained ninja-like attacker could leave their first class seat, and obtain cockpit access.

But I guess it'd be too expensive to provide full pilot separation.