Easy Simple Preventative Waterproofing! Ten dollars. 2020 Plus.

 
To show how more crud is definitely accumulating, the photo on the left was taken in March. The photo on the right was taken in May. 500 miles ridden between photos.
 
...My thoughts are that the gap is on all the bikes not for letting water out but for letting wind in. If there is wind blowing in there then water will dry out...Some of us use leaf blowers to dry our bikes after washing...same effect!.
 
I don't see how anyone thinks water can flow up hill to drain out. There are already drain holes in the bottom of the housing where the water goes out. It doesn't need any more drilled.

That opening can't allow anything to drain out being higher than the drain holes. You can see the salt and sand left behind AFTER the water drains out the BOTTOM holes already in the housing, not the opening in the welting that allows it to enter. Putting the seam of the new longer welting at the top stops that. It doesn't let any rain or road grime in.