It isn't so much an issue of some dealers not selling enough, or cannibalization of sales that motivates GM. In fact one could argue that either of those situations boils down to a big problem for the dealer and not so much for the car company. When dealers order for their lots they put in order forms for a number of different cars with different features so that they can more or less cover the spread of what are likely consumer preferences (Maybe you have to convince someone to take a different color here or an option there, but basically you have something pretty close to what they want. To do this it is necessary to keep a fairly large number of vehicles on the lot at all times. If they can get rid of a dealership, then that's one less lot full of cars that the company needs to provide an assorment of cars for. The truth is that one dealer can probably meet your town's needs, but it will involve a faster moving inventory, which is better for GM.
If this isn't clear, let me set up an example. Suppose there are two Chevy dealers in your town and they each carry three models. Model A has an invoice of $15,000, Model B of $20,000 and model C $25,000. The dealers both like to stock around 30 of each (usually the spread of cars is 2 months worth of sales, so it could be more or less depending on how fast your car market is) in order to have a good selection on hand. So on average each dealer needs $1.8 Million dollars worth of merchandise on the lot at all times. GM can't wait for the money to trickle in according to sales of the car, so they finance them, they lend the dealer money through their financing arm that pays for the cars until the dealer can pay off the financing. So while the company has cash flow, it's based on credit until the dealer can make the sale and discharge that liability on the balance sheet. They're essentially fronting $3.6 million dollars of merchandise in that town. So what's the reason that they want to take out a dealer? Because they don't want to have to front $3.6 M when they know the single dealer could keep offering a proper selection with only $1.8 M and more frequent ordering. Apply this on a nation wide scale and you can see how this would ease their need to devote resources to supplying this credit.
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http://www.intellichoice.com/carBuying101/HowDealershipsWork