Usability and accessibility are separate, though related. Yes, websites are easy to get to and start using, but that won't help a badly designed web app be less

Usability and accessibility are separate, though related. Yes, websites are easy to get to and start using, but that won't help a badly designed web app be less bad or mitigate the impact of weird, non standard conventions and interactions. A well designed desktop app will be more pleasant to use than a badly designed web app despite the added overhead of installation. And of course, things can be done to cut the friction of installation down to a minimum, e.g. using fully self contained binaries, distributing zipped executables instead of installers, and including robust self update capabilities (on platforms without package management).