This is always an excellent point. Looking at the decade as lasting from 2000-2009 is a purely arbitrary conceit created by fallible humans. Looking at the decade as lasting from 2001-2010 conforms to laws of physics intrinsic to our cosmos. The latter is clearly the correct approach, despite the fact that most fallible humans have incorrectly identified decades as consisting of those years when the little number in the tens place doesn’t roll over.
Not really. You see, the first decade had only 9 years in it, the years 1-9. Or else there’s an implied Year Zero of the Common Era (since even Jesus wasn’t really born in Year 1, or Year Zero, and we’re well beyond calling it Anno Domini) where nobody did anything at all interesting worth talking about, a complete write-off in world history. Or you can consider it a one-time rounding error somewhere in the past, it almost doesn’t matter which decade.
Either that, or you adopt on a Quixotian, prescriptivist attitude and go around “correcting” people whenever they refer to “the 90s” or “the 80s” as “a decade”. Which is fine with me too, I find it funny.