Think about longevity. Whether I'm writing, drawing, making music, or anything else important to my life, will I still be able to view/edit that data in 20 years? Granted, even with offline-friendly software I may need to port it to new platforms and make sure I back up the data, but at least I have a chance. The data I still have from 20 years ago is far more precious to me than I thought it would be back then. Data that can only be accessed through an external party is ephemeral and it will inevitably disappear one day.
You are right to talk about the data first and foremost. 20 year old software is very rarely useful (yes there are exceptions) but the data is often just as valuable as before or even more so.
That is why I much prefer a web app that lets me download my data in an open and well documented format to a desktop app that locks my data into some obscure proprietary format or even into a particular device.
