IAD is not that far out of the way. It is just a horrible place. Neither was JFK (another old UA route). The total elapsed travel time was similar, and it gave a longer chance for sleep. Don't forget that SCL is east of MIA (not by much) and EZE is a nice chunk east of there.

Back to the OP, I once connected @ MIA from Lan to AA and it was a horrible experience - F on Lan to 2 cabin F on AA. From the check-in and security zoo that took forever and a day, to AA lounge matrons who almost didn't want to permit entry with a F OW ticket (Lan), I hate the place (among other trips from a near misconnect due to MIA immigration, to a near missed flight with a security line that was so long I had to have a AA EXP who was on the phone for his own misconnect put me on with his agent to see if she could protect me, too (I have no AA status) - I just find any way to avoid MIA with a South America to domestic connection in the early AM. I don't know if AA through DFW would serve you better. However, much of their SFO/DFW service is on MD80s. I have no info on CO.