The challenging part is to provide the back-end. Get massive storage, with proper checksum integrity, at a cost effective value and with flawless security.
A single command (and minor variations around it):
aws s3 sync <name_source_dir> s3://<name_target_bucket>
get's you all that. It can be easily mapped to an intuitive user interface by a software development boutique in weeks.
And because that is the difficult part, it seems for the first 8 years of their history that was solved for them by AWS, according to their own reported history.
Sharing a folder or file from Dropbox is no more intuitive than using the S3 console. Looking at the very colorful history of security issues around Dropbox [1] it's certainly not safer.
So why would a user pay more for storage costs than with a cloud provider, with notoriously less security? Does a tiny layer on top one or more cloud providers justify the add-on value for the user or the current valuation of 9 Billion dollars?
[1] "Criticism of Dropbox": https://en.wikipedia.org/wiki/Criticism_of_Dropbox
side note: I'm filled with regret for every document that I authored on Dropbox Paper. It didn't last for long, but there was a time that it was the official wiki for a company I worked at. Its not impossible to export, but never with full-fidelity.
I've not really used the S3 console but, at least on macOS, sharing a file from Dropbox is as simple as "right-click on the file in Finder, Share..." then enter an email address or create a share link from the popup. Or I can right-click and "Copy Dropbox Link" if the direct link is good enough.
Is the S3 console simpler than a right-click?
aws s3 presign s3://awsexamplebucket/test2.txt --expires-in 300
How is this easier than "right-click, pick a menu option"?
(If I want an expiring Dropbox link, it's there under 'link settings' in the pop-up. Admittedly only has date granularity but still there as an option.)
Not that I like Dropbox too much. I'm going to switch to my self hosted solution. But it's certainly not as easy as using Dropbox, even as someone who uses AWS since 2012 frequently.
"...Half-a-billion people stored files on Dropbox. Well, sort of. Really, the files were in Amazon’s cloud. .." [1]
[1] "The Epic Story of Dropbox's Exodus From the Amazon Cloud Empire"
https://www.wired.com/2016/03/epic-story-dropboxs-exodus-ama...