those long warranties bring down wages as that type of work yields lesser pay. you end up w/ younger, less experienced techs that stay. COULD hurt perception(didn't mean to imply that) if said techs do less quality work--it's a good bet.
there was an local article or study a while back here locally supporting such a claim. most of the stealership techs i've seen are in deed very young. the experienced guys typically go independent or open their own shop.
I will say this. The guys in the dealer have up to date factory training. With vehicle systems changing every couple of years that's a lot to keep up with. In a dealer a tech can specialize in one make. At an independent shop the techs, if they are going in for regular training are getting generalized information that would apply to a majority of vehicles.