By your reasoning, it would be wrong to complain about cars which have a speed limit of 40 miles an hour, since "the nearest competitor" (bikes, horses?) would still be still worse.
Also, the driver can't use a laptop, but because of the driving activity itself, the driver is not so easily bored. Boredom is the main reason why I power on my laptop on a plane. Also, the other passengers can easily work on a laptop, watch a movie and so on. If you've got a 3G connection, the passengers can also go online - even I as the driven, I often find myself checking my emails while driving, plus I'm connected on Skype for the whole ride. The only problem here may be the comfort of your car, as you'll have problems having a laptop in your lap if the car is a small one. I also never get anything done on a plane when trying to work, especially because I don't have Internet connectivity, so I would rather listen to Podcasts or read a book and when driving Audio books are great btw.
So, related to comfort, I always prefer a couple of hours in my 10-year old Audi A6 over a plane ride. If the car ride is longer than some threshold, like the mentioned 6 hours, I do agree that a plane ride is better.
Why would you ever do that? If you don't care about your safety, at least make sure you're less likely to kill others, please...
Note: Number of lanes paints a much stronger picture, but I can't find a good map of that.
I'm quite fascinated by the interstate system; I think it's one of the more amazing things this country's managed to accomplish in the last century. I admit that I draw my sample set from just my experience -- I grew up in the SF Bay Area, go to school near Chicago, and spend summers in Seattle, so all my travel is huge E-W distances -- but I think even if you're right, you have a good argument for N-S rail systems and there's still enough E-W travel that air travel is still quite necessary.