I think the SaaS model applied to desktop software would run into some problems. I think people would react with hostility if their desktop software stopped working once they stopped paying a license fee, even though they may very well accept this model for browser-based apps.
I think the sparrow guys charged too little for their email client, personally. People will pay for beautiful and quality, productivity tools.
It's unfortunate, but the desktop model just doesn't pencil out in many places in todays world. That means that stuff will end up in the cloud and on the web, not because that's where it can be best implemented (I far prefer a good thick client over a good web app) but rather because that's where the economics force it. And users will end up being held ransom by a cloud service anyway.
It's a funny economics problem.
The economics of the upgrade-train desktop model are bad and getting worse, and the market will supply less of that. I may be proposing something not inherently appealing to you, but I think increasingly that's where people smart enough to build great software will focus their efforts.
iCloud/Dropbox has really set the expectation that syncing should be free.