It's easy to say and pretty much easy to do. For the average user, the catalog of applications available is good enough - except you want to use some really complex/professional software.
"Only Windows" application are mainly commercial, I guess? For Open Source/FOSS/Free Software there is absolutely no reason to be "only for Windows" + developing software is easier on Linux, as Windows has a different and closed-sourced architecture. This "only Windows" (for non-Microsft apps) hype has its roots in the facts, that Windows has monopole about OSes and because of that, it's not lucrative enough for a company to code complex (maybe paid) software for a minor user group - even if they do, mostly OS X comes before Linux. Also Windows hold an even stronger monopole speaking of working-environments in businesses -
For most people, it's not an option.
Yes, for most employees it's not an option to switch to another OS, as software they must work with is supported only on Windows. Nevertheless, some average employee doesn't have even the choice to decide about using Windows or not - if Windows (OEM) is on the working computer, than Windows will be on the working computer, privacy issues up or down - except for those paranoid corporations with ultra-secret business secrets - only there the change is needed (although why would they use Windows in first place - even XP, Vista, 7, 8, 8.1 tracked some statistical data and had the possibility to catch some more). But this doesn't mean that an average person at his/her home with his/her PC cannot switch to Linux/BSD - yes, it requires some time to get in with something other than Windows but it's certainly possible and an option.