Those are good points. To be successful, any business web application will have to have a very good explanation of why their service provides an extremely high level of data security. To solve the potential of a web app. going away, the application needs to provide a very easy way for the business of extracting all their data in a form where they can easily deploy that data using another application.
The seismic shift is being made possible partly by Salesforce. Their users tend to be the top tiers of management and sales, and they have completely sold that level of users in so many small and extremely large corporations. This means that the rest of these companies' users are easy targets to shift over to web applications if a meaningful service is made available as the upper management fully understands web applications and their value. Having that new set of web application operate seamlessly with upper management, and feed financial data back to upper mangement for analytics is a very big deal.
The seismic shift is being made possible by reliable secure data platforms. We plan to use S3 and have an xml option for extracting all data. For businesses, the ROI of using web apps is growing tremendously. The cost of providing all that infrastructure in house is huge, and the applications they currently use are very poor at collaborative work, which is what most businesses need. So the door is open right now in a way I've never seen before for business web applications that work well and connect to other applications. The key is interconnecting all of the companies data - something most companies have only dreamed of until now that the web approach can provide in an elegant and low-cost way.