One-time fee means that you pay more money upfront. Plus you have to install it, which is both a security issue and unnecessary hassle if you don't like it and need to uninstall it. If you change your machine you need to reinstall the app. If you need it from more than one location (home, office) you need to pay for more licenses. Support is way more complicated for desktop apps. And then next year they release a new version of the app, and you need to pay for the upgrade, so it's pretty much the same dynamics as annual subscription.