In my view, AJAX will likely not replace desktop applications; but it will give birth to a new breed of enterprise AJAX grade software applications as services or better know as (SaaS). Most will be productivity tools, collaboration and business portals. Some will no longer be seen on the desktop at all. I think this next evolutionary phase in software will be less of an all-out exodus from the desktop than a moderate realignment, with applications thriving in the environments that best suit their purpose.
AJAX-faced web applications can follow you, like all web applications can. Most of us have at least one friend who traveled around the world and kept in touch via email with hotmail or yahoo e-mail accounts. In fact, these browser-based email interfaces helped drive the adoption of email for personal use. These days, using a more traditional html interface for business class e-mail use isn't really practical anymore. Outlook Web Access (OWA) (although it had a great AJAX interface) didn't really catch on for some reason, but Gmail and Zimbra are seeing uptake and Zimbra is aimed at replacing Outlook and Exchange altogether; and delivering all the UI through the browser. To Microsoft's credit, its Live.com initiative is giving birth to a new AJAX-enabled email client that will likely be much nicer to use than Hotmail if it's anything like OWA. If you look at the CRM world it seems that web apps have already taken over, Salesforce.com is an obvious example that has been delivered through a web browser for years.
AJAX-based web apps are the logical choice in applications where up to date and shared information is absolutely critical such as in logistics, accounting and CRM systems. Timely business data is a key component of modern businesses and web applications are data centric by their very nature. AJAX just puts face on the service orientated architecture application that does not cause tears of boredom as users wait for pages to refresh between mouse clicks.
Applications that rely on web services and disparate data sources such as mapping, or to a greater extent GIS, that combine multiple enormous data sources to remain relevant and useful. Also, many Service Oriented Architecture (SOA) based enterprise systems are perfect candidates for the distribution and usability benefits of AJAX apps.
Obviously all the benefits of software on demand, SaaS, and ASPs still apply, because the application is still delivered over the web and in a web browser. It would be silly to waste time going over the benefits of browser based apps; however, it's important to note that AJAX makes browser based apps more usable. These usability benefits can be measured in terms of time savings when interacting with faster user interfaces.
The world of desktop applications will benefit greatly from the use of web services and SOA. We will continue to see more integration of these two worlds. Already we are seeing desktop apps consuming rich content facilitated by the Internet such as iTunes, and desktop apps that will integrate with online with AJAX-based services like Live.com and MS Office.
In terms of penetration and adoption rates for new apps (web based), AJAX takes the cake. No other technology has entry barriers as low - all you need is a browser and an Internet connection. Furthermore, many AJAX applications will experience the benefits of network effects far faster than client apps since adoption can take place that much quicker.
Look at Google maps - no user could have all the mapping, images, and business/address listing data on their local desktop. Further the concept of mashups, which is a website or web application that seamlessly combines content from more than one source into an integrated experience, would not work if users had to install a plug-in or something every time they want to combine different data sets. Even if a client application had the data they couldn't keep it as up to date nearly as easily as web-centric app.
Collaboration will be incorporated into applications like never seen before. Systems like Writely, once adoption picks up, will change the way we think about office type applications. From the start all applications have a common platform, the browser, and speak common languages (XML, HTML, etc.) this means that as these small applications evolve it will be much easier to integrate them. Currently MS Office integrates well with itself and if you use SharePoint it works pretty well for collaboration. However, this depends on massive amounts of software to be installed on each client and pretty intensive server infrastructure to boot. But web apps should improve this experience and AJAX will make those web apps usable.
Using the power of the data services on the web combined with the power of the rich AJAX UI’s and real-time collaboration applications will change forever. Documents and data sets will seem much like leaving evolving content than a static view or piece of paper.