Wi-Fi? Besides, what exactly is your point? People might have had access to SMS first, but it wasn't a good solution for much of the world. It seems to me that you're trying to argue that we haven't made much real progress in the past decade and that you're trivializing WhatsApp in order to support that point. It seems to me that you're focusing entirely too much on the longevity of specific apps and not looking at the changes that those apps actually represent.
As for Dropbox, it's sort of like the iPod of cloud hosting services. Yes, hosting services like Photobucket existed. They were mostly bad and slow and hard-to-use and special-purpose and didn't offer much free storage.
As for why WhatsApp specifically was such a success over competing apps, it's hard to say, but speculation is that a large part of it was due to their supporting many legacy phones.
Anyway, the main differentiator vs. SMS is simple. At the time, many carriers charged for SMS separately, even if you had a data plan. WhatsApp did not. It just used your pre-existing data plan or even Wi-Fi. You therefore didn't even have to have a data plan to use it if you could get Wi-Fi. On top of that, it was just a far more polished experience than SMS and didn't rely on MMS for pictures, etc.
This sounds trivial and it's easy to write it off, but this is why the app came to be used by half a billion people around the world instead of SMS.
Sure. But everyone got SMS before they got data (and Whatsapp).