You are right about historical reasons but I believe wrong about the ones you have supplied. I believe the primary reasons are more likely to do with distance and communication.
When you were a factory in the 40s and 50s pumping out cars you were focussed on the manufacturing aspects and your business was located in a single location - ie Detroit. The idea of trying to manage a network of stores across the country when communication was by post or expensive phone calls just simply didn't make sense. It was more cost effective to outsource that work, in exactly the same way it makes more sense to outsource things like accountancy, IT services and legal in many organisations.
Selling cars is a different business to making cars. And there is no guarantee that if you are good at making cars you will be good at selling them. The original idea was that these dealer were to be your customer interface, and ideally do a better job than you could.
The fact that manufacturers obviously decided that wasn't working at some point in the past and tried to open dealer owned stores is a sign that that system broke down.
Today, communication is almost instant, manufacturing processes and methodologies are more flexible than ever before and there is far more information available to your average consumer. A lot of what were "meant" to be the value adds of dealers are gone. Dealing directly with the customer allows you better control of your brand and if done well will increase profitability by removing a stage in the process.
But it can't work for every product. You still won't be buying your softdrinks direct from Coke or your shampoo from Unilever. Because in these situations the dealers (ie supermarkets) still make more sense.