Er, non sequitur. Just because someone isnt using OpenOffice (or libre), doesnt mean theyre not using a word processor. Just because youre not using Audacity, doesnt mean youre not doing sound editing.
Your statement would only be true if it could be shown that all Open Source software is inherently better at the task it sets out to do than proprietary; and we can look at the state of graphics drivers on linux (noveau vs nVidia blobs) and see an immediate, disproving counter-example.
If there were no opportunity costs, that would be correct. Because there are opportunity costs, Open Source only has to be better than whatever else was an option given the resources available. If I am an aspiring developer living on $ramen/week, I cannot afford to purchase high end %dev environment, %office software, %operating system, and %hardware, but I might be able to purchase %hardware and use open source for the rest, and thus still be productive.