@thejwt Don't forget the change in the update cycle. From 2004 - 2009, 600cc and 1000cc sportbikes were getting major updates every other year. From 2009-2020, the R6 had one minor update. The ZX6 had two. The GSXR and CBR600 are the same bike.
The 1000cc class has received more updates, but they're still on a 5 or 6 year cycle instead of every other year.
Whenever there are rapid update cycles, there is incentive to buy new stuff. When it's 10 years later and the only difference is that the new model has LED headlights, the engine is neutered right when you get to the fun part of the RPM range (an ECU flash will fix) and there is now a 20lb brick in the exhaust (that will just get removed immediately) there's much less incentive to buy a new bike.