If I am not mistaken (and I might well be) most books these days are not manufactured on acid free paper. Given that, they will probably last 50-70 years at best. Unless you treat them to prevent decay, and store them in a temperature and humidity controlled environment, or sealed.
Having said that, I agree with you that longevity is key. We don't need material to be lost because data portability isn't available. However, given increasing power, the ready availability of tools today that let you strip out the text from proprietary formats, and virtualization (who cares if you can't read the file if you can virtualize a machine that can?) I am not terribly worried.
There are good and bad reasons for not buying an ebook reader, but I don't honestly think this is one of them. (And yes, I am a convert. I have had a Sony PRS-500 for 2.5 years, and just bought a 505.)
Every successful person has had failures but repeated failure is no guarantee of eventual success.