The answer: it depends. It depends on what type of barbecue you buy. As with any other manufactured product, there are high-value products and there are low-value products. Don't forget to consider the long term availability of replacement parts for any barbecue you're considering. If you're buying a barbecue that's built in China by a company nobody's ever heard of before - it just might be hard to get parts for it down the road.
No doubt in my mind about this: the barbecues that age better than any other barbecues are Weber. A customer was in the store the other day looking at the Weber barbecues. He couldn't decide what colour to get. I told him that he had better pick a colour he likes because he's going to have it for a long time. He said, "Well I don't really expect to get anymore than eight years out of a barbecue." I stared at him. What the hell did he just say? I obviously hadn't been clear enough with him when I was talking about the Weber quality. "Oh no," I said, "... if you don't get at least ten years out of this barbecue, I'm going to consider my entire life a failure." Now it was his turn to stare. I told him: "Barring a catastrophe, extreme misuse or the everyday grilling of 'Agent-Orange Burgers', this barbecue is going to last much, much longer than eight years." He ended up taking the black one.