I'd be willing to bet that a large amount of the most used applications out there are proprietary, in many cases because there is no comparable OSS equivalent.
Your argument would be significantly stronger if you actually gave examples here.
Photoshop - Gimp can't handle tablet input on any system I've got (lags horribly behind input), and is generally slower and has a horrendous UI which frequently puts windows half off-screen and sometimes cuts off text.
Maya / 3DS Max / Whatever - Are you really going to claim Blender is any good? Functional, sure, but that's about it.
Windows - Linux graphics drivers are often an absolute joke, so Windows really is just about the only real PC gaming option. And don't claim it's because graphics cards are closed hardware - then why hasn't an open-hardware group made anything competitive? Their only major success has been the Arduino (a BIG success, but rather lonely).
OSX - Linux OSes, even Ubuntu, have all had clunky UI and seriously lacking normal-user help documentation. Last time I used Ubuntu, I had to resort to the CLI inside of 30 minutes to correct a permissions error the UI could only inform me of. Try telling most people how to do that.
Outlook / Mail.app - Thunderbird's the biggest contender, and has many things right... but while the new UI is much nicer, I ran into many display bugs in less than an hour of use, one of which resulted in my deleting an email I did not have selected (when I didn't hit delete).
MS Office suite / iWork suite - OpenOffice has a UI lagging behind even MS Office, and in many of my uses has failed to have a working updater and lags behind my typing frequently. And then there's the preferences window... most people simply cannot understand 90% of that, and will be turned off the entire application by its mere existence.
Half Life / Quake / Unreal Tournament / PC games of any kind - ?
Console games of any kind - ?
McAfee / Norton / AVG - ? ClamAV? Brought my system to a crawl last time I used it. Not that McAfee / Norton are much better there, but...
And in the other corner, we have:
Firefox / Chrome, Pidgin / Adium (though Pidgin has a bad install process and a nasty UI for most people), Eclipse (though it's slow)... I know there are more, but I'm having trouble coming up with them right now. Just woke up.
I had generally thought the major ones were self-explanatory, and this is rather long, hence its initial omission.
Your argument would be significantly stronger if you actually gave examples here.