They don't make their service revenue primarily from brand new cars, for the obvious reasons you've stated. It's when the timing chains, water pumps, head gaskets, piston rings, clutches, etc break, or the preventative maintenance to keep them from breaking (Audi timing chains, 10x $$$ if you don't replace them on schedule because of the interference engine), including the regular fluid flushes, battery replacements, air filters, etc. 80% margins on the little things add up :)