In the event of a blinded vision system, such as sun glare, the vision system should be able to recognize its reduced capacity. That involves warning the driver and relying more heavily on radar/sensor fusion.
Like others have mentioned the entire field of sensor fusion deals with this problem. It is a very challenging problem to solve but it can be solved and it has been successfully used in spacecraft, aircraft, fighter jets, phones, AR/VR systems, and undoubtedly many others.
A basic approach is to have an uncertainty (or estimated uncertainty) for each of the sensing modalities. Then you use the uncertainties to weigh each sensing sample when deriving your estimated quantity (i.e. vehicle velocity for example). Assuming the uncertainties are correct, the resulting estimator can have variance lower than estimators deriving from a single sensor modality. Of course tuning sensor uncertainty values is a difficult problem in sensor fusion (and much more so when distributions are unknown) but it is definitely doable.
Repeating Elon's claim that sensor fusion is impossible/not-doable is entirely wrong. It is a technology that powers many different applications but its definitely not an easy thing to implement well.
There's a huge amount of literature on how to do sensor fusion in the presence of misbehaving or disagreeing sensors.
Moreover, production vehicles don't get entire classes of redundant sensors when the engineers think they can do without. The cost optimization and haggling that goes into production vehicles is insane.
