Maybe they could have pulled it off. It'd have certainly added a layer of moral complexity, with Cap weeping as he bashes a brainwashed octogenarian in the face with his shield. But yeah, it's probably best that they went another direction with it. As for the RDJ stuff ...
Disagreements About Civil War Almost Ruined Everything
Robert Downey Jr. practically is the MCU. Hell, the next Spider-Man film is all about people trying to live up to the standard he set (which was also the plot of the last Spider-Man movie). So it's bizarre to think that a pay dispute almost kept him out of Civil War and nearly caused Marvel Studios president Kevin Feige to quit entirely.
RDJ's contract was up after Age Of Ultron, and really, there was no reason he had to be in Captain America 3 at all. But Batman v. Superman was coming out, and Marvel wanted a similar hero vs. hero film that could dunk on it so hard that the backboard shattered. So they asked Downey back for a small role, but he asked for a much bigger role (and a much, much bigger paycheck), which turned it into Civil War, aka Avengers 2.5.
Enter Marvel Entertainment CEO Ike Perlmutter. When he heard about this, he went ballistic and tried to get Downey written out of the movie. Marvel Studios president Kevin Feige, the mastermind behind the whole MCU thing, was apparently so pissed off by this that he threatened to quit.
Just on a story level, that would have had a ripple effect across the rest of the MCU. Civil War would have been severely different, as I mentioned. But you also lose the Spider-Man / Tony Stark mentor relationship. Considering how the plot of Infinity War and Endgame unfolds directly from the events of Civil War, those are now different films too. And would Downey have stuck around for more films if things had turned ugly?