At least you're consistent with it. There are a lot of people that seem to gripe for the sake of griping or because they jump on the same bandwagon as everybody else that is doing it and it drives me nuts.
You are definitely right about the Mustang bulking up before 1974 and I don't particularly care for the looks of them either but they definitely "resemble", if you want to call it that, the earlier cars more than the later cars.
They sold a lot of the 74-78s though.
They did sell a lot of little Mustang IIs..... I was a kid in high school back then and remember the period very well. I earned my gas and car money working part time pumping gas in 1976-78 at a local gas station on the high way near my parents home. The fact that there were no mechanics working the bays was an added benefit to me and the other guys working there. We had free access to the lifts. Great for working on our cars.
But the other front line look at things I had was the impact higher gas prices had on car design and used car pricing. The muscle cars were cheap and available in a BIG WAY in my youth as second hand cars. I cringe as I think of the cars we DESTROYED as we cut holes in the doors for cheap speakers and then also just allowed them to decapitate because "kids" don't have the coin to properly fix up and restore cars. Everything from Big Block monsters to small block screamers that today fetch big prices at auction were throw away gas guzzlers deemed to be worthless to anyone but kids and gear heads back then.
Those little cars were the desirable models. That '74-'78 Mustang sold well with it's little motors because it could achieve good gas mileage and in a world with gasoline over $1.00/gallon for the first time vs prices under 50 cents just a few years earlier.......the belief at the time was we'd never see "muscle cars" again. Given the new emissions rules and changes required to meet 'em nobody saw the day when it'd be possible anyway.