I disagree. Cars are not fine. First, when people say cars, they are referring to civilian cars, not transportation cars.
Regardless, you are committing a fallacy. "In some cases, cars are helpful, therefore, cars are not our worst idea". A very simple way of showing that your conclusion is false is by just giving an example of having just 2 ideas, 1 of which is obviously worse than the other, even though that worse idea has some pros. Sorry, I know this is pedantic, but I just wanted to point out the fallacy first, in general terms.
But now to your point itself. Say we never invented cars and let's also say that that implies no ambulances (which it does not, we could have ambulances in a sort of public transport, ram ambulances, bike ambulances, etc). The amount of deads that cars cause is orders of magnitude larger than the amount of reads that ambulances save. I don't have data on this, I think everyone agrees with this. So, still, net positive.
So yes, I will repeat. Cars might just have been the worst idea we have ever had.