IIRC there was a major downturn in the number of bike sales in the 80's...so there will be fewer vintage machines to pick from, by the time you get to the 90's the market was being flooded by the super cheap WM bikes. Many bottom level 60's and 70's bikes were of a reasonable quality and have survived, by the mid 80's those were starting to be made offshore and the quality dropped drastically. I rebuilt a couple of Huffy Baypoints, one from the early 80's made in the USA and a late 80's made in Mexico there was a world of difference in the frame quality. I have a brand new Huffy cruiser that is basically a bicycle shaped piece of metal. I see no way that it could make it 20-30 years...even if it was never ridden!
Aaron