It's not.
The confusion here stems from Microsoft's maddeningly obtuse naming conventions. Leaving mainstream support only means that Windows 7 won't be receiving any new features or product tweaks, such as the impressing-sounding DirectX 12 gaming technology slated to launch with Windows 10. You won't be able to call Microsoft for free help if you run into an issue with Windows 7 either.
Once a Windows desktop operating system leaves mainstream support, it enters the extended support phase--the very same support phase that Windows XP found itself in from early 2009 until its death earlier this year.
You'll still receive those oh-so-critical security patches during extended support, meaning that while Windows 7 won't be in active development beyond next January, it won't be tossed to the wolves of the Web, either. Hotfixes will still be provided, too, assuming they're security related. (Business can sign up for an extended hotfix support plan if your company wants hotfix support for non-security issues.) And those security updates will be coming for a long time, too: Extended support for Windows 7 lasts until January 14, 2020. Heck, commercial PCs with Windows 7 Professional will even continue to be sold for the foreseeable future.
You can read Microsoft's Support Lifecycle for Windows if you're still curious about how this all works. (You can also find information about the other products reaching various End of Support statuses on January 13 in this Microsoft Support post.)
So take a deep breath. Relax. Don't panic. Windows 7 isn't being left for dead until years from now, even if the timing of Microsoft's Windows 10 events (including the one scheduled for January 21) directly coincide with Windows 7 End of Support milestones. In fact, here's three ways to buy a new Windows 7 PC today--and yes, downgrade rights from Windows 8.1 Pro still work just fine.
Editor's note: This article originally published on July 10, 2014 but was updated January 12, 2015 as Windows 7 exited mainstream support.