Do we stop using seatbelts until they are 100% effective? Was it a mistake to start putting seatbelts in cars before it could be proven that they absolutely wouldn't kill anyone?
If you are trying to attain maximum safety for a belted passenger, then no, current passenger seatbelts are not optimal. This is clearly evident from the far better seatbelts and driver safety devices used in motorsport.
Current seatbelts represent a tradeoff between safety if you use it, and how annoying they are to use (and therefore, how likely people are to get annoyed and not use them).
Say someone gets hurt from a seatbelt. If it's designed exactly as required by regulation, there is an immunity for the company (IANAL so I don't know the legal term for this)
IF you try to make it better, and do make it better for most cases, but worse for some edge case, then you have a liability issue in that your 'negligence' caused injury that wouldn't have happened otherwise.
The stance on civil liability in America often stifles innovation in many areas regarding life safety. Tesla is learning this quickly.