Hey, there's another benefit to lightweight cars: environmental impact. All else equal, less weight makes them handle better, stop quicker, accelerate faster, use less gas, and fewer materials used in construction. Reliability means you can keep driving it for many years, not scrap it and buy a new car, which reduces your resource footprint, making it even more "green".
I think the data suggests that passenger cars aren't as bad as many people think they are. To put into perspective, passenger cars make up about 40% of transport emissions, which make up 30% of total emissions. So that's 0.4 * 0.3 = 12% of total emissions. But even that overstates things, because 90% of car emissions comes from 25% of the cars. Of course these are the old cars. So all the new cars put together (which far outnumber the old cars) make up only about 1.2% of total emissions.
For comparison, meat and dairy production account for about 15% of total greenhouse gas emissions. So if everyone stopped eating meat & dairy we'd do more to reduce emissions than banning passenger cars outright. And we'd all be healthier too.