Any improvement _over_ 99.999% is impossible. Saying something like this is completely detached from reality. People _do_ buy cars that crash once a year. All the time. And often the defects in the system aren't even related to self driving functionality. Further, most drivers who get in a car have records far worse than that.
No, on basically all accounts. In the US there is a fatality every 75,000,000 miles, a injury every 1,250,000 miles, and a reported accident every 550,000 miles on average [1]. From a "reliability" perspective, we can reasonably assume that at any given time, you will probably crash in less than a minute without control over your car. The average driver probably averages ~40 MPH per unit time. So, on a minute basis that is one fatality per 111,000,000 minutes (99.999999%, 8 9s) a injury every 1,875,000 minutes (99.99995%, 6 9s), a accident every 315,000 minutes (99.9996%, 5 9s).
Even the worst component of the driving system (the driver) is solidly over 99.999%. And every single hardware component is vastly better than the driver. The Pinto, a classic example of a death trap, only resulted in a hardware-induced fatality something like every 1,000,000,000 miles, or using the minute basis above a fatality every 1,500,000,000 minutes (99.9999999%, 9 9s). The person you responded to is correct, 99.999% is unusably, criminally bad.
This thinking is absolutely going to get someone killed. It’s also why Tesla faces so much criticism.
Any improvement under 99.999% is unremarkable because the end product is unusable. No one is going to buy a car that crashes once a year.
Disengagements should be unheard of if Elon wants to continue selling FSD.