Level 5 is unachievable and unnecessary. There will always be some small number of locales that confound the system. But if you can't use autonomy a few times per year, who cares?
When we get to level 4 the game is over, it's just an adoption curve at that point.
I don't know how "unnecessary" it is if we're talking snowed roads, country roads, roads that haven't been painted in a decade or longer, or if we're talking about autonomous cars without a wheel.
Under those conditions, it's pretty much Level 5 or bust. But yes, that doesn't mean Level 4 cars won't start getting adopted, especially if the autonomous systems are just another "feature" on a regular car (hopefully EVs in all or most cases).
Having to turn off self-driving features will be like losing cell service. It's not what you paid for, it can be mildly inconvenient, but you get over it and on with your day.
When we get to level 4 the game is over, it's just an adoption curve at that point.