That's a huge assumption. There's no real indication that people would stop using Windows if Office existed in Linux.
If that were the case, then porting Office to OS X would've had a similar effect, don't you think?
Ultimately I believe the reason Microsoft doesn't bother porting to Linux is that the market-share is just not worth the cost of re-writing for another platform, and keeping it updated.
Additionally, as a compromise Office 365 runs on browser, which further decreases the incentive to port to Linux. Since you can get a similar experience using Office 365 on Linux, that said I won't be so disingenuous as to imply that the experience is as good as a native application.
In conclusion, Linux has a very small market share, so making a native application for it might not be cost effective. Additionally, Microsoft might see that market share as "served" regardless due to their online Office 365 web application.