All valid points, but I will simply defend the other side. Not playing Devil's advocate, but I just believe the thoughts you expressed are framing a lot of people's negative fears for the show...
The downside to this is that your gonna either:
1. Run out of ideas very quickly. Or go back to the whole "TGUSA is ripping off TGUK" comparisons again. Or, we'll get some bizarre challenge that even the die-hard TGUSA Fanboys will have to agree, is the "Jump the Shark" moment. ("............The final challenge is sticking a freshly dead pig corpse into the car and let it rot in there.")
Sure, I guess. Though, to be fair, I also don't think a society hell-bent on destruction and violence (reference our love of demolition derbies, monster trucks, or any other motorsport with a high probability of things going poorly) will turn down a show in which three guys simply hoon around in cars 10-20 times a year. Especially cars in which many of us have fond memories of our own hooning around. Given the relatively low number of niche manufacturers in this country, I can't see how they are ever going to fill enough airtime with legitimate reviews. Even TGUK struggles to do this in 12-15 episodes per year, and the list of "built-in-a-shed" manufacturers they have to choose form is much more extensive than ours. I would actually be surprised if they didn't start highlighting personal customization jobs a couple times a year (a la the Red Victor 1 and Hill Climber-in-the-kitchen episodes of TGUK), given the clout of our grassroots society.
2. Start to piss off a rabid fanbase even worse than the Morris Marina Fanclub. I can already see this happening given the number of specific forums for mostly every single car or marquee ever made out there. And we're talking about the USA alone. Heck, some of them are outright nutso if the TGUSA boys destroys a particular car.
I was actually kind of annoyed when they destroyed a VW Caddy during the first cars segment, so I know where this is coming from. However, no one ever said they have to outright destroy every car, or even do it in an outright senseless manner. We are a country equally attached to the feel-good story. Want to test pick-ups? Spend a week helping Habitat for Humanity. Want to test the sportiness of cars from a specific decade? Contact an organization like ICY Racing and have the teens work with the presenters to prep the cars for a race. Will the vehicles be the victims of mishaps? Absolutely, but when it's all done in the name of helping someone else, it will quickly be forgiven. I don't think it's hard to come up with a means of entertainment which is at least forgiven by most. (Remember the end of the America trip, where the cars -- well, two of them, at least -- were given away in New Orleans?)
3. Run out of cars to mess up. I know they shoot in California, but there has to be a limit in how they acquire cars.
At most, even if they do ridiculous challenges in which the car gets destroyed, they will have to source 30-60 vehicles per year. I live in a semi-rural setting and drive 10 miles to work each day, and I would suspect that the number of cheap/pathetic/interesting/charismatic/commonly-owned/envied/entertaining cars I see on a daily basis just during my commute could keep the show going for a year or two. I don't see any way they run out of cars anytime soon.