It's not only about file storage. Ubuntu One offered a Web service to programmatically access the files. Imagine you've developed an application that interfaces with Ubuntu One and is mission critical for your business.
I bet you can count the number of people who did that on one hand. They're not shuttering the service because it was popular. They are open sourcing the code though, so you can run your own version and not change any code.
Not essentially different from if you developed your business application on a PaaS which went bust or raised prices. These things happen unless you keep them in mind
