"Wind chill is irrelevant unless you're discussing exposed skin." .....that's why I said "without considering the wind chill..."

On my 2013 the mirror is mounted to the glass in the usual way. Beside that is the block that holds the rain sensing camera. The block is quite small compared to the system in the 2017.
On the 2017, there's a huge area that's taken up, and all insulated from the warm air from the defrost. If it was higher and wider, it would be a non-issue....the fact that the top 2-3" of the drivers side wiper passes over that area is the issue. The wiper squeegies water over that large cold patch of glass and the water freezes there, then on the next pass it squeegies more water and it freezes in a layer on top of the first. Keep doing that and shortly you have a 1/2" thick pad of ice that the wiper is thunking over on each pass, being a general pain, and causing the top part of the wiper blade to ball up with ice as well.
The winter folks know that it you want to make a hockey rink in your back yard, that is pretty much exactly how you do it. Apply a layer of water to a cold surface, let it freeze, then add another layer, and another.....
Just grabbed a few pics from the web... First is what I have in my 2013. Next is a 2017, and then a CX5..... I'd say the rain sensor on the 2013 only covers a space about 2" wide.....likely the heat conducts (inducts...? Somebody will correct me, I'm sure) through the glass enough for it to not be an issue...or, because the sensor is out in the open it warms up with the defrost air and transfers that heat to the glass. Either way, the last 2 pics should give you an idea what I'm talking about.