Yes, dealers are one channel for authentic parts, but it's the manufacturer that's required to make them (the people who sold you the car aren't building parts in their back lots); lots of places would be willing to sell them (because selling parts and repairing vehicles is obviously profitable). And even with dealers and 1st party hardware, 3rd party produced parts are extremely common - right now - easy to get, and usually cheaper.
Your theoretical problem has proven not to be one in reality.
In reality, the consumer could see some improvements from a consolidated dealer/manufacturer. It avoids the round of finger pointing you sometimes get between dealers and manufacturers when it comes to warrantee service (the same reason many prefer to buy an iPad at the Apple store). And it's ridiculous to pretend manufacturers/dealers are carrying some heavy cross in terms of selling replacement parts - it's a profitable part of their business, and they spend a lot of money advertising it (as opposed to trying to shirk their responsibility or something, which they might try if it was some burden).