What happens to the cassettes that shops replace after a few thousand miles? I'm sure there's a reason to do so, but I'm also sure I could be happy using cassettes that shops consider worn out for the rest of my life. I've been riding about 4000 miles a year for the last 4 and a half years, and while those miles are spread out over a few different cassettes, I have yet to replace a cassette because I thought it was worn out. I'm more conservative with chains, replacing those every 2500 miles or so (based on wear measurements), but the cassettes seem fine to me.