Hi,
I'm a software developer as well
For most businesses the questions will be:
"If my application on the web service dissappears how does it effect me?"
i.e. Am I out of business if the web application crashes, the company goes broke, my internet connection is unavailable for a few days?
I am a huge believer of web apps so I'd suggest letting customers have access to their full data as a download. Even if customers need to pay a little more for the extra service it removes one of the biggest fears.
Now hoe they access their data once the have it is another problem as well