The Dart & Valiant were pretty solid cars - and were bulletproof with the slant six. The Japanese cars of that era had major issues with rust, which finally began to improve by the late 80's. My dad's '74 Datsun was rotted out within 5 years.
First, I owned a 1975 Opel Manta in the 1980's and never had any trouble getting parts for it from the local Buick dealer; not bad for a car that they had stopped selling ten years earlier.
Second, the "so-called" Plymouth Cricket was built by Hillman in the UK, not Mitsubishi.
Third, your statement that those "captive imports" were "every bit as good as, if not better than" the domestic models you cited is 100% your opinion, and has no basis in fact. The ones built in Japan or Germany were just OK, but the British and French built ones were just... awful. Ask anyone who was unfortunate enough to have owned a Sunbeam Arrow or Simca 1204 back then.
I owned an 1969 Opel GT. I purchased it from a Buick dealer before they even hit these shores. Had to wait 8 months for the delivery. I had ordered blue, but the one that arrived was gold. Nice thing about it was that it was number 608 off the assembly lines, and therefore had no emission control devices. Price for the Opel was around $4500.00 at the time.
In the next 5 years I put another 3 to 4K into the car, and in early 1970 it was featured in one of the car magazines of the times. They called it "The Blue Max". Yes, I went with the color I wanted. Modified the drivetrain, and went to a Vega rear end and 15" tires instead of the 12" that it came with.
As my family grew, I needed money and sold it to a young rich boy. The car lasted him just under a month before he totaled it. As to other Opels, I owned a 1966 Opel Kadett and drove it for over 200,000 miles.
"Again, I should stress that every one of those "Captive Imports" were every bit as good as, and usually measurably better than, the competing American models sold at that time; cars such as the Dodge Dart/Plymouth Valiant, Ford Pinto and Maverick, and the Chevy Vega."
Whoa there!!! The Dart, Valiant & Maverick were COMPACT cars, not at all in the same class as the 'captive imports' you mentioned, and IMO the Dart/Valiant were virtually bullet-proof vehicles in their class, & the Maverick wasn't bad either. Some of the captive imports were very good cars; others not so much, & many suffered from premature rust-out. None were as good as the sub-compacts available nowadays, although many had nice styling that's been lost in many of today's subcompact offerings.