It could also vary widely based on whether or not advertising was a business's greatest problem. If your biggest problem was that not enough people know about or had tried your restaurant, Groupons probably did a lot of good.
If your biggest problem was that your food wasn't really that great or your prices were too high or you were in a terribly inconvenient location or your staff was unbearable, then getting more people in the door doesn't really help anything in the long run.
I think you're right, and I think the facts coming to light, even just in this discussion, prove that the vast majority of businesses trying to make magic happen with Groupon were just not sustainable. Groupon has ridden the notion that small business owners could get a "web 2.0" surge to success. What's happened is that entrepreneurs have burned through their startup budgets, and ignored sound business sense in favor of fadish practices. Really, Groupon has bled a bit of startup capital out of the economy. It's not a zero-sum game.
It's like how online journalism is collectively wringing their hands about ad-blockers and crying, "But what about the CONTENT?" Well, most of what passes for "content" these days is regurgitated click-bait non-sense, recycled in a dozen places, and it should all just go away anyway. Cry me a river. In both of these cases, the "good" notion of capitalism is working just the way it's supposed to.
If your biggest problem was that your food wasn't really that great or your prices were too high or you were in a terribly inconvenient location or your staff was unbearable, then getting more people in the door doesn't really help anything in the long run.