If we're talking about the Model 3, wouldn't you also not be able to know your speed since the speedometer is on the center console? I feel like that's pretty significant for most drivers.
The main cluster reboots in 30 seconds or so, so you’re not without the speedo for long. The speedometer isn’t a safety-critical feature over that time period.
It is if you need to know the speed limit during that 30 seconds and it causes you to get into an accident. I've strongly felt like Tesla drivers were the biggest assholes on the road, and things like "how fast I'm going doesn't really matter" is a pretty good confirmation of that.
I have never been in a situation as a driver where I needed my speedometer to prevent me from having an accident. I also am perfectly capable of maintaining a steady speed for the 30 seconds it takes the car to reboot. I expect most drivers are. You can determine the speed of your car pretty accurately by looking at how fast you’re moving by objects outside of your car.
It’s perfectly legal to drive in a car without a working speedometer, and many people do.
I don't care about legality. I care about safety. And everybody assumes they're an above average driver. You should do everything possible to minimize accidents, otherwise you're not driving defensively.