When I used to do up Dysons (usually DC07s) for resale, the amount of motors that were arcing out due to shorted armatures was very high, and on pretty much every one, there was plenty of carbon brush length left. I think that so many motors (the YDK type) are ruined long before the brushes wear out, mainly due to users not keeping the filters clean. What you have done by changing out the filters should help prolong the motor's life.