Yaris rear wheel woes

Looking at the rear wheels I noticed that there was something relieved:

 A closer look revealed that it was kept relieved by all the sand and small stones that probably got inside from the road.

 To remove all the dirt, you'll need to detach the whole piece by unscrewing the screw that's directly below.

Be prepared to repeat from time to time, as the front is not tightly coupled with the rest, so all the dirt from the road will collect inside.

Who's the engineer that came up with such a bad design in a car with one of the most advanced engine?