The M-Disc claim of 1000 years is based on a few estimated tests and while they mostly hold up, we have objective proof that glass left alone over 1000 years does not change to the point that if you wrote something on year x, time-travelled with your reader to year 1000x , the reader would be unable to read it.

I don't expect to crack 90 years myself, but my great-great-grandchildren might want or need whatever data there is on that glassdrive, even if it is only pictures and not something important like legal documents.

The 'rest of your life+' claim is simply my shorthand for saying that while the glass will likely last for thousands of years, how long the readers exist that can read that particular glassdrive and interpret its data (ANSI or Unicode text, PNG or PDF display capability) may or may not last as long as the glassdrive itself can (see Voyneuch manuscript which exists, but we no longer can read and interpret it).