Re "why didn't mystery men enlist in their costumed identities" -- well, when a football player enlists, he doesn't show up for basic training wearing his uniform and helmet; when a baseball player signs up, he doesn't bring along his bat and say "don't bother giving me a rifle, Sarge, I'll take care of the enemy with ol' Slugger here!"
Actually, that'd be pretty cool. But you see my point? If you want to keep your unique approach and distinctive look, you don't sign up to wear a uniform and take orders.
On the broader topic: a while back, Steve Englehart wrote an interesting (but badly editorially mangled) four issue miniseries called Big Town, about a parallel Marvel universe where Reed Richards (as well as Tony Stark and Hank Pym) shared advanced super-technology with the world, and what sort of hyperfuturistic place it was to live in a decade later. In spite of the flaws, the book is a great favorite of mine. If Marvel prefers Zombie Earth to techno-utopian Earth, that's their business...