My responses to your opinion:
1. They weren't commonly fielded or at all by US troops (M14/M1A excluded)
So? How / why does that matter?
2. M14 went out of regular service 50 years ago
Again, how/why does that matter?
3. None can be purchased under $1000
Today. I bought mine when they were $500 or less...
4. G3/FAL were select fire so most kits coming into the country had to get new receivers/barrels/etc
The M14 is also a select fire weapon. Which is why there is an M1A...
5. No method to inexpensively manufacture new semi-auto models for the US market
Since people seem to be buying the "expensive" models why would the market change??
6. .308/7.62x51 is expensive relative to other common military calibers (5.56, 7.62x39)
There is no free lunch, you pay for what you get, and the 7.52x51mm provides a different performance envelope than the 5.56 or the 7.62x39mm.
7. You see similar characteristics with 7.62x54R chambered semi-auto rifles
Relative to what? cost? availablity? define your standards, please...
Compare to AK/AR stuff
1. AR style rifles used to be prohibitively expensive, but now some of the most inexpensive semi-autos on the market
I never considered the AR prohibitively expensive. It wasn't cheap, and IMHO wasn't worth the cost for a long time, but that was 50 years ago...
2. AK style rifles were dirt cheap for decades
After the fall of the Soviet Union, they were, before that, they were "unobtainium" simply not available at any price, and the same for their ammunition. After the collapse of Soviet communism, East bloc nations sold LOTS of soviet design stuff, dirt cheap. That lasted for some time, but today has pretty much gone away.
3. Plentiful surplus ammo for super cheap
Again, true for a while, nearly all gone today....
4. Common models well under $1000
Today. Tomorrow who knows?
Lower powered rounds firing lighter bullets produce lighter recoil. If that matters to you, it matters. Doesn't matter to me.
6. Familiarity in the US with M16/AR-15 and "commie" AKs
Familiarity does aid market popularity. A popular look and a popular price goes a long way in market sales.