I don't think showrooming would be catastrophic to the industry. If test drives are important to selling cars, manufacturers could pay dealerships to keep their cars around on lots for that purpose, or come up with a meaningful incentive structure to reward them if the test drive leads to a sale (you can incentivize the customer to help with the tracking, too; just give them a big enough coupon). And alternative manufacturers could sell through the Internet or at their own dealerships, at their discretion.

And certainly used-car dealerships aren't going to evaporate under this scenario either.