I'm sure it's the type of thing in an Elon meeting he would say "Surely we can think of a solution better than relying on drivers to provide meaningful input to the desired outcome?"
Yes, in theory it would be best to have the perfect speed without intervention but there isn't one perfect speed for the conditions.
The speed the driver wants changes from driver to driver and might depend not just on how much rain is coming down but on the amount of traffic on the road, proximity to pedestrians, eye sight and time of day and any number of other factors.
It's always going to be better to have an automatic system that can be tweaked by the driver because everyone is different. The more easily the driver can do that without thinking the better.
