There are plenty of Dropbox competitors out there, more are coming out all the time, and some people are even claiming that this alone is proof enough that Dropbox is done.
This argument ignores two factors, one of which being inertia. As with any incumbent, offering a service that's just as good or even slightly better is not going convince people to switch.
The friction of having to move all your files to a new cloud, install several new apps and so on is enough for most people to not even consider it.
Dropbox revealed during MWC that one billion files was being uploaded to the service each day. That presumably includes files that are updated, not just new files, but it's still a staggering number. Mega recently boasted about getting 125 million files in a month.
Dropbox also announced that its 100 million users had used the apps or the websites on 500 million devices.
Not even Apple or Google can hope to achieve that kind of reach any time soon, not to mention the large number of scrappy startups or me-too services that keep popping up. While the two may be able to reach as many or even more users, they don't see the kind of activity Dropbox does.
Size and inertia are not enough though, wait long enough and someone will come up with a service that is not just marginally better, but which is several times better and people will switch.
Luckily for it, Dropbox is not relying on size alone, it's also adding more features that go beyond cloud storage, things like photo sync and more recently document previews.