I hear what you are saying but not everyone has so much money. I had to upgrade to a D200 for a wedding and didn't drift far from A mode with exposure compensation & minor tweeks. My D70 was adequate but occasionally created unreadable shots and wasn't reliable enough. I would have liked to had more time with the D200 but, as always: diapers, kids clothes, rent/mortgage, electricity, etc. outweigh a new camera without an immediate $$$ benefit.
It also depends on the subject matter. I certainly wouldn't use a new camera exclusively if National Geographic called me up to do an assignment on Tuareg's in Mali (although I'd be willing if they read this!).