smasha: You should not have to change out your derailleurs or chainwheels for a long time; certainly they should outlast several chains and cassettes if you keep them properly maintained. "Something bad" doesn't usually happen to bike drivetrains, unless you whack a derailleur against domething; they usually just slowly wear out. Cleaning, lubrication and timely chain replacement will go a long way toward keeping that from happening for many thousands of miles.