The arithmetic of the pedant - a decade is just a group of 10 years (no starting point required for the basis of the year number) so it's convenient to group them as 2000-2009, 2010-2019 and so on. 2001-2010 is just ugly.
Quizmonster, a decade ago when people were arguing over the millennium, someone wrote a letter to The Times that has stuck in my mind. It was something along the lines of: 'It doesn't matter when the millennium starts, it commemorates nothing at all, certainly not Christ's birth; but people are happy to greet the new millennium in 2000 because it's like the odometer in your car: it's when all the numbers go round'. I think that's still a good reason to enter a new decade today.
Dennis Exiguus should have allowed for a Year Zero when he calculated the Christian calendar. (Perhaps the concept of zero was not well understood at the time?) Pol Pot did, not that it did him much good.
"Pedantic arithmetic" and "ugly numbers"? Good grief, I thought mathematics - of all the disciplines - was by definition free of any such 'human' considerations! But what the hey!
"Mathematics, rightly viewed, possesses not only truth, but supreme beauty -- a beauty cold and austere, like that of sculpture, without appeal to any part of our weaker nature, without the gorgeous trappings of painting or music, yet sublimely pure, and capable of a stern perfection such as only the greatest art can show."
Of course not...YOU'RE the one arguing with him! Do you imagine HE would have said that numbers can be (I quote you) 'ugly'? Indeed, in the very Russell quote you offer, he refers to mathematics' "supreme beauty" and denies our "weaker nature" any part in the discipline. I'm sure he would have been perfectly happy to acknowledge that decades end at the number ten, not nine.
As I said earlier...What the hey! And I'll leave it at that.
by the way, jno is right - a decade is a period of ten years. They start whenever you like. Cambridge advanced learners dictionary gives the following definition..
'a period of ten years, especially a period such as 1860 to 1869, or 1990 to 1999', so yes we have just started a new decade.