I have a soft tonneau with the rails mounted to the inside lip, not on top. It suits my needs fine (mostly by serving as an "out of sight, out of mind" deterrent), but water does come through that small gap to reach the drop in liner. For the most part, the channels in the liner are enough to keep most of what is in the bed reasonably dry. I can't remember off the top of my head if there is a tailgate liner or not.
I had a previous truck that had a drop-in liner and matching tailgate cover. The tailgate cover was held on with plastic push-in fasteners, so no metal corrosion woes. Some of them did eventually break...20 years later...because I pulled the cover to fix the latch handle.
The truck after that had a cap and bed mat. Had no issues with that setup other than the cap limited my cargo options at times (bulk mulch, etc.) and enabled things at others. No tailgate cover, it was just starting to rust out along the bottom seam when I sold it.
I guess it depends on what your long term ownership intentions are. If you intend to trade or resell it later, that's one thing. I've run 4 trucks into the ground, and not one was gotten rid of due to bed rust. Frames on the other hand... The first one I owned actually had a rusted out bed back by the tailgate when I got it, I simply reinforced it with some angle and kept using it.
FWIW, I am in the New England "rust belt."
I had a previous truck that had a drop-in liner and matching tailgate cover. The tailgate cover was held on with plastic push-in fasteners, so no metal corrosion woes. Some of them did eventually break...20 years later...because I pulled the cover to fix the latch handle.
The truck after that had a cap and bed mat. Had no issues with that setup other than the cap limited my cargo options at times (bulk mulch, etc.) and enabled things at others. No tailgate cover, it was just starting to rust out along the bottom seam when I sold it.
I guess it depends on what your long term ownership intentions are. If you intend to trade or resell it later, that's one thing. I've run 4 trucks into the ground, and not one was gotten rid of due to bed rust. Frames on the other hand... The first one I owned actually had a rusted out bed back by the tailgate when I got it, I simply reinforced it with some angle and kept using it.
FWIW, I am in the New England "rust belt."