It's easy to look at the world today and say that web applications have won. This is web developer arrogance. Stupidity is to think that web applications have won because web applications are superior to desktop applications. Smarter, but probably still arrogant developers would point to web applications as disruptive technologies. This involves admitting that web applications are inferior, but good enough, and present enough other "cheaper" advantages to compensate for their inferiority.
To understand why the "web apps have won" claim is dubious. There are definitely a lot of awesome web applications out there. Many of them were created back in the mid/late 90's, The "features" of these applications were the key to applications, not the user interface. Now these days, most of these web applications offer APIs/web services/RESTful interfaces/whatever you want to call them. In many cases it is possible to build desktop applications that tap into the same features as these web applications. However, this was certainly not the case 10-15 years ago.
So if APIs make it possible to build desktop apps that offer the same features as popular web applications, why haven't people switched? The first most obvious answer is inertia. If you are used to accessing Google or Amazon on the web, that's probably the way that you will always use it. Something else to consider is that for many web applications, it does not make sense for them to offer all of their features through APIs because it hurts their core business. This is most obviously true for advertising based companies like Google, Yahoo, and Facebook. Their web applications not only provide very useful features to end users, but they serve ads that make money for the companies. If all of their users switched to using desktop applications that only offered the features with no ads, then the companies would lose their revenue streams. Their business is connected at the hip with their UI, so it is in their best interest to make sure people use their UI -- which is a web application.
However, there are other very successful web applications whose main revenue does not come from ads. Their business is distinct from their UI. E-commerce companies like Amazon and my employer, eBay are obvious examples. For example, eBay offers trading APIs that provide almost all of the trading features of eBay. This is particularly true for selling on eBay. This makes sense, as eBay does not need a seller to use the eBay UI to sell something, as the UI is not what makes money for eBay. As a result, around 50% of all items for sale on eBay come through 3rd party applications built on top of the eBay trading APIs. The vast majority of these (especially the popular ones) are desktop applications. Give people a choice, and a lot of people choose desktop applications.
For another interesting example, just look at Twitter. This is a company that came into an existence after web APIs had become the norm. So Twitter has provided a comprehensive set of APIs since early in its existence. Further, they have not pursued an advertising model that would marry their web based UI to any revenue streams. So they have kept their APIs in sync with their features. For example, they recently added list and retweet features to their site, and added them to the APIs at the same time. As a result, there are a huge number of desktop applications for accessing Twitter. Indeed, Twitter says that 80% of their traffic is from APIs -- either desktop or mobile applications. For most Twitter users, there have always been feature-equivalent desktop alternatives to Twitter's web based UI, so many users chose desktop applications over the web.
Finally, let's look at one more example: existing desktop apps. There has been an incredible amount of money spent on creating web applications that provide similar functionality to traditional desktop applications: email, word processing, etc. Heck, Google has spent a lot in this space just by itself. These are useful applications, but it is rare for people to choose these apps over their desktop equivalents. In most cases these apps try to go the disruptive route, i.e. don't try to be as good, but good enough and cheaper. They have had little success so far. Of course, inertia is a valid argument here, too. The one case where there has been success is GMail. In my opinion its success is not because people like it's web UI over a desktop UI, or even that the web UI is "good enough" and cheaper. No, it's success is because it has offered innovative features over other web and desktop based alternatives: fast search, stars/labels, threaded conversations, etc. Even give all of that, many people still choose to use desktop clients to access their GMail (I'm definitely not one of them.) Anyways, once again it's the features, not the UI.
I am not going to sit here and claim that desktop wins over web all the time. I'm not arrogant or stupid enough to make such claims. However, be wary of claims that web apps win over desktop apps. One could argue that with the preponderance of APIs (especially spurned on by mobile apps) and the popping of the advertising based web 2.0 bubble, that the future will hold even more opportunities for desktop alternatives to web applications. Maybe web applications have jumped the shark. So don't put up with web developers who insist that web applications have won (especially if they try to extrapolate this flawed argument to the mobile world). They can go on and on about technology, standards, interoperability, etc. Just remind them that it's the users who matter, and when given a choice, the users do not always choose web applications. Time to polish off your MFC and Cocoa skills!