As in the game is a fad, or the franchise itself? I'm inclined to believe that yea, this game is more of a fad, but the franchise itself is going to remain strong. This game would not have become the sensation it became if it wasn't pokemon. It was the perfect marriage of a strong and popular franchise and a really neat concept. I think Pokemon should be Uni's top priority if it isn't already. Even if Pokemon Go fades, the fact that it became such a hit in such a short amount of time is a testament to there still being a fervor for this franchise (and ESPECIALLY the first generation of Pokemon). I knew Pokemon Go was going to be big, since I knew Pokemon was still popular, but I never imagined it would have done this great in such a short amount of time. That Pokemon can pull that many should tell Uni that this franchise is here for the long run, if the fact that it's already 20 years old and still going strong hadn't told them that already, and people will still care for it even if Pokemon Go fades away.
That said, I don't think Uni has to be in a rush to get something out. I'm sure they would have loved to have something to cash in on while the iron is red hot like it is now, but Pokemon (the franchise as a whole) will remain very popular regardless and the fanbase will still be there.