Second: I realize everyone’s experience is different, but the three times I’ve had the misfortune to interact with Dropbox shared files (once in a corporate setting, twice with files shared by acquaintances outside work), it just didn’t work. So the meme that Dropbox is a reliable mechanism for sharing to others confuses me.
Obviously your experience is your own, but I have yet to have a Dropbox sharing event fail for reasons other than user error (typically by unsophisticated relatives or coworkers, so I'm not suggesting at all that this was the root of your problem).
We should probably both be cautious of overgeneralizing from our own experiences with Dropbox, but the general presence of "it's easy and it works" even among the nontechnical suggests that your experience is atypical for some reason. Did you ever figure out the cause?
After all, it was once totally conceivable that a multi-million dollar company would be founded on the basis of, basically, a device driver.
We're still doing that sort of thing, its not a new phenomenon that some crack in the design/methods/ethics of a technology vendor is exploited by some un-controllable third-party. Gone are the days when Microsoft, by way of example, would have waged war and just built-in local- and remote- peer discovery, rsync++, etc. in response to this daring challenger to the hegemony.
I guess, I really do sort of miss that. Dropbox really sucks. I only use it because my friends do.
I can't imagine Apple or MSFT ever offering something that would truly replace it - anything either does is likely to privilege their own OS over any other, and Dropbox has no reason to do that.
The other reason that services like Dropbox shine, at least for me, is that by providing a cloud-mediated sync solution, you don't have to coordinate a synchronous conversation between the devices in question. That's valuable, especially since it enables easy sync with > 2 devices (which also opens the door to selective sharing).
That always-there aspect is also how Dropbox became the de facto mobile file system for iOS (and Android, maybe? I'm guessing -- I'm an iOS user). It's also the default mechanism for sharing files too big or numerous for email (e.g., Aunt Sally's holiday pictures).
My tl;dr is that while Dropbox's initial value prop might not be earthshaking, it's done what it set out to do very, very well for me since I started using it 8 years ago, and I'm very glad to pay them for their service.