I'm not saying this will be the next Dropbox, but dismissing products so offhandedly because you underestimate the average user's aversion to "writing a few lines of Javascript" is silly.
Op got downvoted a lot (and perhaps deserves it because of the tone of his post) but makes a valid point.
This is nothing like dropbox. This is barely a hello world app on the Twilio platform. I have little doubt he could write a tutorial to walk people through it in an hour two two.
This would be a good analogy if you dropbox was literally just provisioning a user and setting an FTP password. And if it was, Dropbox would hit scaling problems on almost day one.
This won't hit scaling problems because it doesn't do anything... Twilio's API has built-in API calls for all this stuff and the webhooks can (and should) be hosted on AWS Lambda or Google Cloud Functions for pretty much free with automatic scaling.
It's actually a feature. It saved you one day of your life. And if this free service just disappears some day, you are just one day away to build your own. You got your number from Twilio, there is no lock-in.
Personally if I built this I would probably open source it. Then anyone can run it. Although I suspect this is just a marketing ploy for http://aworldforus.com/ or they built it for their own use and decided to release it.
BTW, anyone interested in a tutorial? I can create one.