Yeah, I don't think it's unreasonable to question its long-term appeal in its current form. It's truly fascinating and already at this point it brings people together, but it is quite limited in what you can actually do. Whether or not this is enough to keep people interested after the first novelty has worn off is definitely a legitimate question.
Even then, though, the app is used by so many people already that a drop-off is just to be expected and it definitely has enough breathing room that even when this happens it will likely remain very, very healthy and extremely profitable for quite some time.
What's interesting to me, though, is that the core mechanics aren't actually all that social, since you can't interact with other people in-game. This definitely is the one important area that needs improving: The game needs trading (this is what made Pokemon so extremely popular in the first place [they definitely should not allow online trading, though]) and fights against other trainers, so that the people who care have something they can actually use their Pokemon for.
And another thing: Battery packs aren't $100, you can get them for a tenth of that.