When we are actually have cars that sufficiently close to what humans can do in terms of range of condition, I suspect the level 5 definition will be updated to be more like:

Can drive in approximately the same set of conditions as a human (professional driver?), possibly being unwilling to drive in certain (seldom encountered) conditions where most humans would, but offset by being willing to drive in conditions few people would.

Importantly, the car must avoid completely giving up on driving mid trip (as opposed to deciding "too dangerous, turn around and go back at next opportunity"), unless conditions are comparable to those in which a human would give up mid trip (which are pretty limited, as humans seldom just stop and give up on the road unless the car is broken down, or fully stuck. At worst, in some really bad conditions, humans may pull other to wait for the storm/extreme-fog/etc to blow over.)

Or perhaps at that point we won't need the definition anymore. It becomes a bit arbitrary and market-y at that point. Assuming that we don't end up with a single vendor.

"My BMW still drove during the snows in February, but my neighbor's Tesla said it wouldn't drive 2 of the days." "Yah, but mine doesn't insist that the windows are perfectly clean and pristine before starting a trip!"

We probably should never have had level 5, but split L4 into a couple levels: heavily geofenced/restricted vs. relatively unlimited applicability with some restrictions.