I have also operated single-man and 2-man cockpits and agree that 2-man was easier with the systems in play at the time. A nav saved my life too, and another couple almost cost me it. Pilots come with variable levels of ability and so do back seaters.
What has changed since the 2nd and 3rd generation fighters is the quality and extent of automation and avionics, which will have developed even further by the time Tempest is fielded. For example, F35 has an autoland capability, Harrier did/does not. The F35B/C has an auto-eject capability that is active in some flight regimes - that would have saved a lot of lives on the Canberra, Sea Vixen, Buccaneer and the like. Simulators have improved to the point where zero-flight time training is a realistic option (Harrier had a 2-seat variant for training, F35 does not, etc.) and to the point where genuine mission rehearsals are entirely possible.
Everywhere you look, designers are trying to remove the prospect of human limitations, and part of that will be removing humans from the cockpit where possible - I don't think there will be any aviators who like that idea (I certainly don't) but it seems to be the fiscal and technical reality we are heading towards.