I agree times have changed. Despite Watergate cynicism in regard to government and government agencies hadn't yet affected all of television program yet when the SMDM was made, It was therefore easy for the audience at large to still accept the idea of a good guy doing work for a benevolent (so to speak) government agency. Today I think that would be harder to sell because many have become quite cynical.
That isn't to say a revamped version of the SMDM couldn't be done. I think the short lived series Now And Again
came close, but they chose to focus on the hero's family connections and strayed away from the action-adventure and intrigue aspects of the story.
Again it all comes down to execution. If you set it up right and deliver it in just the right way then you could still make it work. It's too easy to say "this won't work because no one will buy that anymore." There are too many examples of that being said until someone comes along to successfully challenge the assumption.
Westerns don't sell, but then we get Dances With Wolves
and quite a few other good westerns since. Pirates don't sell anymore and then we get Pirates Of The Caribbean.
Superheroes are strictly for comic books and Saturday morning programming. Really...