The demise of PFE started way back in 1965 when they Douglas rolled out the first DC-9. Weighing in at 79,999 lbs, it just squeaked under the FAA >80,000 lb/ FE requirement rule.
No doubt, the third set of eyes have been valuable and has proven itself many times. Many of the accidents could have been averted if there was a third guy on board. They also make a great tie-breaker in the cockpit.
As far as SO goes, great as a heavy crew on a two man cockpit aircraft but on the a/c that requires FE, they tend to get "seat distraction".