It's more me saying that they've introduced too many dragons than as any reason why it wouldn't be a dragon type. Kind of tired of 'em really. (IMO does denote subjectivity, not logic, after all)
Form and function. That is my answer as to why I don't think Froakie will become dual type. At this stage, it doesn't lend itself to acquiring a second type (ie, I do not see any features that could change to take advantage of some other elemet) I've seen an animation of froakie attacking with the white stuff behind its head, leading me to concur that it's based off the foamy secretion some frogs make to house their eggs/tadpoles. Chespin, being the spiny nut pokemon resembles a squirrel or chipmunk than it does a hedgehog, but its earthy brown color lends it a possibility of an earth type (as with turtwig). It's description of being always optimistic doesn't fit something that would likely become a dark type. And fennekin could very easily be another pure fire type. It has a frail looking form, which one would think would pronounce a creature reliant on its special attacks and abilities.
If you'll notice, gen 5 had one dual type and the other two were single types in their final forms, so it is not without precedent.