I feel the true danger is in the "uncanny valley" of automation - it's good enough that you don't need to pay attention 98% of the time. People underestimate tail risk, ignore the manual etc.
My guess is that you're right: level 5 will take about as long as it takes to get to photorealistic rendering - never quite there.
If you apply that reliability percentage directly to time driven (inaccurate, I know, but for the sake of argument), a system which takes care of 99% of driving problems will still abandon you to chaos for 18 seconds out of every 30 minute drive. This is an eternity in a moving vehicle where milliseconds can draw the line between life & death.
The only systems safe enough to allow fully automated vehicles in the near future will have to be on dedicated lanes or roads, probably with additional safety mechanisms on the roadway itself.
My guess is that you're right: level 5 will take about as long as it takes to get to photorealistic rendering - never quite there.