A coach who's had previous experience who wasn't an abject failure will command more than an untried coach regardless of who turns out to be better. It's the same in every industry, an untried manager in a bank who gets a promotion into the role will command a lower salary than an experienced manager brought in from somewhere else. if the untried coach proves to be a success then their next contract is on significantly improved terms (eg Clarkson at Hawthorn), does that mean they're suddenly a better coach? No, it means they're being paid in accordance with their experience.
Supply and demand comes into it because there's only a finite pool of experienced candidates to draw upon and not everyone is prepared to give a rookie a chance.