I think it really is a matter of if we should put comfortability of taste over physical health. I'm admittedly not up to date with how much nutritional value these things have in detail, and I guess you definitely know more about that, so I will trust your expertise if it comes down to it, but I think having something other than potatoes and beans to provide us with vitamins would be surely beneficial, considering we don't know how long we will be stuck with these ingredients. It could also be some citrus fruit which could also be used to make juice, but I can't think of any fruit without a distinct flavour to it.
I get where you are coming from regarding the caffeine withdrawal, but maybe it's important to know how many people would actually be affected by it on such a level that they would prefer coffee over one more ingredient which can be anything (but I stick with fruit).