Ah, yes but that's a different matter. Scratches etc aren't really a product of longevity as such are they? I can scratch a disc so it can't be read within a few seconds; that's not a reflection on the quality of the disc.
As you say the environmental effects are difficult to predict or measure, and there isn't much real data on the issue with regard to DVDRs yet. I tend to agree that after short periods (6 months etc) most discs that don't read will be as a result of scratches coupled with high initial error rates in the main.
But if you keep a badly-burned TY disc, which still reads, in its box for 10 years, and do not take it out at all during that time, locking it away in a dark, safe place, that disc is still likely to be readable after the 10 years is up. Of course high error rates don't help; what I'm saying is that you cannot just put discs into categories of "will last, won't last" based on PI/PO error rates which may not even be accurate when scanned on LiteOn hardware anyway and there are so many other factors to consider in any case.
As ever, the absence of (systematic) proof of the effects of environmental rot is not the same as proof of absence.