Yeah, but imagine that most people (or dragons that act like most people would act) die much earlier, long before becoming a Great Dragon. That's basically the same problem with Vampire Elders in LARPs: These guys survived hundreds of years, then suddenly they are played by a "most people" kinda guy, act stupid and headstrong and get killed.
Sure, GD's are powerful. But they didn't become GD's by acting like adolescent little punks.