Look at literally every attempt at breaching the cockpit since then.

It always ends in some idiot getting duct taped to their seat at best and beaten to death (SWA) at worst. It’s a different environment. Pax will not sit there.

I won’t go as far as to say the threat is non-existent but I would say current safety measures do a great job with mitigation. I wouldn’t mind seeing it tacked on to new design aircraft but I don’t see a need to have it retrofitted.