Would it have been feasible to profitably transport significant quantities valuable material such as gold from the moon to down to earth with 1960s/1970s technology? If not, would it have become feasible in the subsequent decades?
Maybe. Gerard O'Neill (originator of the now-classic cylindrical space habitat design) proposed setting up a mass driver on the moon in order to provide enough material for his proposed giant lagrange point habitats which would have massed millions or billions of tonnes.
Quite how plausible the mass drive was in the 70s I'm not sure, but it wasn't entirely implausible. The problems of constructing and operating industry on the moon, on the other hand, may actually be insurmountable... moon dust is an absolute killer for mechanisms, seals and people.
You'd need to attach a rocket and guidance system to every payload the mass driver launches in order to inject it into a suitable return orbit, which may also require heat shielding if it is a direct-return-to-earth's-surface trajectory. That also raises some safety concerns, but hey! It was the 70s, people were more relaxed back then.
Collapse of the price of the resource on earth with (or in anticipation of) the first shipments with the resulting economic mayhem.
Maybe? The initial investment would have been substantially greater than the Apollo program. Unless the minerals were super valuable, useful and easy to acquire and return it would take a looooong time to recoup that upfront cost.
There's some scope for the operator to stockpile minerals at the moon side awaiting a favourable market, of course, depending on whether they can maintain a sufficient revenue stream to keep the operation running in the meantime.
Really though, I can't actually think of anything that would be worth mining, refining and sending back to earth. Gold would be singularly pointless... if it were found in sufficient volume to justify what might be a trillion-dollar operation, it would devalue existing gold stockpiles and gold just isn't useful enough for there to be a use for the surplus. Back in the 70s, I don't think there was anything that appeared to be scarce other than oil, and you won't find that on the moon. The only justification for O'Neill's mass driver operation was to construct space habitats, to further support human expansion into space... something also of dubious economic value, alas.
In Heinlein's novel, The Moon is a Harsh Mistress, the inhabitants of the moon are effectively farmers (and support workers) and ship food back to an overpopulated Earth via a mass driver. Also dubious, but would at least be subject to a different set of economic pressures.
With a mature space presence, it might be worthwhile using materials mined on the moon to construct, fuel and power space-based infrastructure (eg. solar powersats which would have direct economic benefits for Earth. Still a very long shot, though.
Destabilization of the earth-moon system by shifting a significant amount of mass from the moon to the earth.
Haha, no. The moon weighs over 7x1022kg. If you could somehow ship a billion tonnes a year, even after a thousand years you'll have only removed less than a millionth of the moon's mass.