That 50 year old Gibson SG is definitely at least a partial case case of survivor bias. All the less-well built ones that came off the production line with neck-warps and dead-frets have long been tossed out by the owner. Though, I bet off-the-line, the ratio of 'well built' to 'not well built' guitars was significantly higher when Gibson was still privately owned.
The other factor is I'm sure back when Gibson and Martin were US-made, there was a certain amount of QA that went into each unit that went out the door. I know some fine men who make their living off craft furniture for the bourgeoisie and every unit is meticulously inspected. I'm sure even if they attained commercial success, they'd ensure a base level of quality that a MNC like Ikea or West Elm probably doesn't have. In the same way you have an emotional attachment to your instruments, those instrument craftsmen/women have a similar bond/sense of pride.
Also, as a woodworker, I can tell you that there are so many contributing factors to wood, ranging from completely obvious spruce vs cherry vs sikta, to more subtle things like where the tree was harvested from, access to water, the way it was cut (quarter-sawn? flat? did the craftsman just select whichever was cheapest?), the way it was cured, in what environment, and for how long. These all contribute to the way they age, the way they cut. It's not unlike wine vintages and variations (which, incidentally, have a fetishized community, but certainly an amateur like myself can identify the difference between two bottles of the same grape and vintage, but of a different region). New SG's are produced in an assembly line with the cheapest acceptable materials, rather than having some skilled tradesman who went down to 4 different lumberyards spending 6 weeks picking the best wood stock for that years production.
I don't think the QA was better then. There's tremendous variability in those older guitars. Arguably, the wood quality was better, but top makers today are still getting their hands on great wood. It's just more expensive.
One of my favorites is a well-played 1980 Tokai. In its day, it was a "cheap guitar", considered inferior to American instruments, but it's magical. It's better than Fender Custom Shops I've played - but I imagine 35 years from now, they'll be much better.
The other factor is I'm sure back when Gibson and Martin were US-made, there was a certain amount of QA that went into each unit that went out the door. I know some fine men who make their living off craft furniture for the bourgeoisie and every unit is meticulously inspected. I'm sure even if they attained commercial success, they'd ensure a base level of quality that a MNC like Ikea or West Elm probably doesn't have. In the same way you have an emotional attachment to your instruments, those instrument craftsmen/women have a similar bond/sense of pride.
Also, as a woodworker, I can tell you that there are so many contributing factors to wood, ranging from completely obvious spruce vs cherry vs sikta, to more subtle things like where the tree was harvested from, access to water, the way it was cut (quarter-sawn? flat? did the craftsman just select whichever was cheapest?), the way it was cured, in what environment, and for how long. These all contribute to the way they age, the way they cut. It's not unlike wine vintages and variations (which, incidentally, have a fetishized community, but certainly an amateur like myself can identify the difference between two bottles of the same grape and vintage, but of a different region). New SG's are produced in an assembly line with the cheapest acceptable materials, rather than having some skilled tradesman who went down to 4 different lumberyards spending 6 weeks picking the best wood stock for that years production.