Splashing up or dripping down, that area isn't the trouble spot that is going to rust out the new fenders just like the old ones. It's the open cavity just below the fender attaching bolt that allows debris to fill up the rear fender support and turn it into a compost heap. GM never implemented a design correction, but it's easy enough to fix.
I fabricated a simple filler plate to cover the opening, and permanently installed it with urethane windshield adhesive. This way nothing can get in the channel, but water can still drain out the bottom.
Of course capping the top really doesn’t do much good unless all the gaps under the fender are sealed as well:
I sealed my front fenders back in 1998 and they remain debris and rust free.