The '70 only lasted one year as well, not to mention each and every year before that. Seemed like '72 - '74 was when Chrysler started to figure out that they could amortize tooling costs over 2 or more years to save development/manufacturing costs.

I would guess (after all, we can only guess unless somebody who was 'there' can tell us the real story) based on how development cycles went and lead times required for styling approval, engineering drawings to be finalized, and tooling to be created in time to refit the factories in the summer of '71, that the '72 grille was decided long before reaction to the '71s, for better or worse, was known.

I would expect that it wasn't reactionary to sales, but that's not to say that it couldn't have been fast-tracked based on reaction to magazines' negative press. It's hard to be sure, but I agree that it is odd that they went to quad headlights for only one model year. Maybe Dodge complained that it looked too much like Challenger? There was a Dodge/Plymouth in-house rivalry happening at the time.