In addition you've got a certain survior bias. Any of the photographs that didn't last got tossed before now. So what you're looking at isn't likely to be a real sample of what 80 year old pictures are like.

Reminds me of those who argue they don't trust RC because it hasn't been used for 100 years. Then go on to point to a 100 year old fiber print trying to prove all of todays fibre prints will last 100 years.

The only real conclusion you can draw is that those photographs have lasted until today. How much is really related to us today? Don't know but I'm doubtfull. Unless you're using the same paper,chemicals and process.