It's a very visible piece of cost cutting and thus loathed by the german car enthusiast.
The W-126 coupes of old had equally big windows back there, that were not interrupted
and had a clever rotating/angling mechanism to slide behind the back wheel well.
This was the way our fathers and grandfathers got their Benz. Above is what we get now.
(But the car is much better equipped on base spec.)
I am not young enough to know everything.