Ravenous hydra and titanic hydra can't be swapped anymore and haven't been able to be for several patches at least. It looks like it was an experiment that went bad; I don't think riot will be making any "free swap" items anymore, it makes them too hard to balance.
There is no reason to make an ad version of witts end because no one would buy it. Physical damage on hit is generally called ad (attack damage) and there are many better attack damage items one could pick up. For example, blade of the ruined king is an item that is doing basically what you want and providing lifesteal and a slow at the same time. Sure, its not taking off 25 armor, but who cares?
Reducing armor is less effective than reducing mr because generally it is much harder for champions to acquire magic resistance. This means that champions in general have lower MR than they do armor, making the flat penetration much more effective. Since all champs gain armor, that shred matters less and less.
Oh, and dealing with the adc? There are at least 3 different actual armor items I would pick up first before I even considered an item like this: Randuins, thornmail, Frozen heart. Hell, If i want more armor, I can get ninja tabi, dead mans, and a sunfire just to "round out my build."
It wouldn't be useful for jungle sustain because there is no way to get the item fast enough. Your first ~2300 gold is in your jungle item and boots. At this point, its generally around level 6/7 and the first towers are starting to fall; you better have your jungle sustain taken care of way before you could even consider picking up an item like this.
Bottom line? There is no point of an item like that.