But 100 years have not passed yet, which makes me sceptical about manufacturer's lifespan claims.

I read an article a year or two ago about early CDs which HAD been around for many years. Discs which were (is this right?) 15 or 20 years old now were flaking and disintigrating in horrible ways.

Sure, media technology has improved over time, but time itself is the best measure of archive media quality. The previous suggestion of keeping multiple copies in multiple locations is good. I'll suggest archives be "refreshed" by copying their data to new media at reasonable intervals (yearly?). If the data is that important, the cost of media and its storage is negligible.