It wasn't Airborne that killed his promotability, it was Special Forces.
The first pertinent quote (from the script) is:
<PERSON> (V.O.)
At first, I thought they handed me
the wrong dossier. I couldn't
believe they wanted this man dead.
Third-generation West Point, top
of his class...Korea, Airborne,
about a thousand decorations, etc.,
etc. I'd head his voice on the
tape and it really put the hook in
me, but I couldn't connect up that
voice with this man. Like they
said, he had an impressive career.
Maybe too impressive. I mean,
perfect. He was being groomed for
one of the top slots in the
corporation. General, chief of
staff, anything. In 1964, he
returned from a tour with Advisory
Command in Vietnam, and things
started to slip. His report to
the Joint Chiefs of Staff and <PERSON> was restricted. Seems
they didn't dig what he had to
tell them. During the next few
months, he made three requests...for
transfer to Airborne training,
Fort Benning, Georgia...and was
finally accepted. Airborne? He
was thirty-eight years old. Why
the fuck would he do that?
1966...joins Special Forces, returns
to Vietnam.
The second pertinent quote is:
<PERSON> (V.O.)
Thirty-eight fucking years old.
If you joined the green berets,
there was no way you'd ever get
above colonel. <PERSON> knew what he
was giving up. The more I read
and began to understand, the more
I admired him. His family and
friends couldn't understand it.
And they couldn't talk him out of
it. He had to apply three times,
and he put up with a ton of shit,
but when he threatened to resign,
they gave it to him. The next
youngest guy in his class was half
his age. They must've thought he
was some far-out man humping it
over the course. I did it when I
was nineteen, it damn near wasted
me. A tough motherfucker. He
finished it. He could've gone for
general, but he went for himself
instead.
Now, for the why any of that is important. 342
You have to look into the history of the US Army a bit. Airborne Infantry was a thing newly developed in World War II. While there were smaller applications earlier, the first major Airborne drops were during D-Day, inland from Normandy. They went, by and large, fairly well. Then during Operation Market Garden, there were more Airborne troops dropped, which went, by and large, again really well. So Airborne and the philosophy and change to Army tactics that were developed from it, were well accepted. By the years before Vietnam, the only officers who would get ('full bird') Colonel were those who had the political backing to continue on to Brigadier General. A Colonel in those years would have served in Korea, and an Airborne Colonel would have been viewed as the more well rounded officer. Not having Airborne wouldn't have precluded promotion, but Airborne was the "Up and Coming Young Officer's Thing-To-Do."
Now let's discuss Special Forces. Like Airborne in the beginnings of WWII, during the years just before Vietnam, Special Forces was only just then being built up. (<PERSON> did so, for many reasons, not the least of which was the glamor effect). The origins of Special Forces were OSS, and various other special groups, usually called Commandos. The problem with the OSS and Commando groups were that the regular senior officers didn't have a clue about what to do with them, or how to use them. (A good movie to watch for this specific thought it The Devil's Brigade. If you watch it, pay close attention to <PERSON> character and how he treats the Brigade.) Those troops and their officers were <PERSON>. They never really had a chance to shine enough beyond single missions to evolve a philosophical change in Army tactics.
So, now we have <PERSON>. He was a golden child. Being groomed for Generalship. On the fast track. Airborne was a way to help achieve that. There was no problem at all with his Airborne training and wings. In fact, they would have made him even more likely to make promotion.
Special Forces? Not so much. In fact, with that, he instantly became un-promotable. There simply were NO generals in or from the Special Forces branch at all.
Finally, when you read the line
Airborne? He was thirty-eight years old. 714