With the military not being terribly popular with a fair segment of the
population thanks in no small part due to the Vietnam War (which was
also draining funds from NASA), namely the counterculture, the agency
also thought it might be best to have a civilian be the first man on the
Moon, which Armstrong was.

Aldrin was a USAF officer who flew jets in the Korean War.

But I also agree that NASA wanted someone who would not go overboard
and start pushing his bosses for more and more complex space missions,
such as lunar bases and sending humans to Mars. Even by 1969 when
the Nixon Administration rejected NASA's plans for more Apollo missions
and beyond, the writing was on the wall as they say.

Armstrong was a very competent pilot who would do as asked and not
rock the boat, as all these decades have shown since.

On the one hand, you can admire Armstrong for not cashing in on his
experience (he turned down millions in advertising offers alone) or
otherwise cheapening his historic accomplishment. But then again I and
I know others wish he had done more to push the space program.
Maybe it would have come to naught, as even Buzz Aldrin wasn't able
to stop NASA from cancelling Apollo after number 17 or get us to Mars
in person, but maybe he - being THE first man on the Moon and all -
might have been enough to make the difference. Who knows, right?

At least Aldrin has tried. BTW, he also got NASA to practice EVAs on
Earth in one of those large water tanks to simulate microgravity. That
is one big reason why his Gemini 12 spacewalk went so well compared
to all the previous EVAs in the series.

And let us not forget that Michael Collins also did his share of space
promotion with at least one book on sending humans to Mars.