A decade is indeed ANY period of 10 years, the same way "a" century can be any arbitrary period of 100 years. You are right, we do not refer to decades as "the 199th decade" for example. We do refer to them as "the decade of the 90s" which is a perfectly legal grouping of 10 years. But you and I both know there are those out there who will twist this strange reality into a claim of acquiescence that does not, will not ever exist.

When someone refers to "the" century or "the" millennium and assigns it a number relative to the beginning of the calendar, such as "the 20th century," then of course we must stand firm that the first century started at the beginning of the first year, and count from there.

You are also right that best chip of the 90s is a hard choice. Here's another favorite.