The automakers(all) in the 70's, in an effort to reduce weight, and thereby improve fuel economy, changed their sheet metal alloys. The new alloys are, in fact, thinner, but stronger than the alloys which were previously used. It was found that O/A welding created a large HAZ which was subject to cracking and therefore the shift to MIG.

By using stitch welds and moving around the panel being welded the bodyman can control the heat much better that was possible with O/A.

In actuality, more brazing was used in body repair, than was O/A "welding".

O/A welding requires a much higher skill level to employ properly than does MIG. In an unskilled hand, an O/A torch can do a lot of "damage" to today's body materials.

Things have changed a lot in body repair in the last 50 years. My dad opened his first body shop in 1946 and ran shops til he retired about 10 years ago. You can bet if today's technology had been available 50 years ago, the shops would have used it. I still remember mixing calcium chloride in a big hopper to produce acetylene for the O/A. There was no bondo then. Lead and spoons were the name of the game for fairing.

Why would anyone want to use 1950 technology when there are better tools available? Don't own a mig. Borrow one, rent one, but get one.

Just my .02