I think doge-dice has a non-negligible cannibalizing effect on just-dice.
DD takes a load off JD that doesn't need to be at JD.
You are correct on the first assertion, but not the second. Even if people are just betting small amounts, they can become inclined to bet more (because they feel like they're on a lucky streak, because they've learned more about the site and want to try with bigger amounts, etc), and there's a certain inertia towards switching from dd to jd.
I very much doubt that DD is positive for JD investors.