I do know you can drive the car while the computer is rebooting (when it's rebooting the screen is completely black and you have to gauge your speed by comparison to other cars the road), so in theory it seems you don't need the computer for basic driving, which would suggest maybe what you say is also possible, if there's a setting for it.
I haven't checked because I like the benefits of being connected — starting and stopping a charge remotely, seeing the state of charge while it's charging somewhere far away, opening / closing windows, unlocking the car, flashing lights or honking horn to locate car, preheating, and coming soon, seeing car's location on a map when it's being driven by someone else, viewing live dash cam (sentry) footage while the car is parked remotely, etc. etc.
I mean I do see your point but I've listed some of the benefits, and life is full of tradeoffs. I'm not saying that the takeoffs that work for me would ever in a million years work for you.
It would also be great if all of the above could be done through a neutral third party or through one's own server. But as a startup that was struggling at the beginning, that probably wasn't the low hanging fruit for Tesla to work on, and there are benefits for Tesla to having the connection be to Tesla. And some of those benefits accrue indirectly back to consumers through safer cars, better accountability for accidents, and better data for future self driving software.