The question, andyb, is, "At what point do we stop basing our evaluation of a player on projections, and instead base it on his established level of performance?"
Now, Franco is 24.303 (per baseball-reference com) years old. Given that, he's not at the point where further improvement becomes a pipe dream (about age 27). On the other hand, he's not that far from it, either. He will probably plateau within two years (assuming he hasn't already - which appears to be related to whether he can adjust to whatever opposing teams have learned about him).
That said, Mitch Walding is essentially the same age (24.288), and he's in AA. So it's not at all clear that he's really an "answer," either. Can he be as valuable as Franco. Yeah, possibly... but as you noted, "as valuable" in these cases doesn't necessarily equate to "valuable."