Microsoft wants all PCs and laptops running Windows 10, but around 40 percent of all Windows machines are still on Windows 7. Some of these are home users, but most are businesses. We won't go into all the reasons why you might not want to upgrade: those are covered in Should I upgrade to Windows 10?.
Windows 7 is still supported by Microsoft
Windows 7 will be supported until 2020, and Windows 8 until 2023. This means Microsoft will carry on providing security updates, ensuring that those operating systems are as secure as possible.
Note that you must have the specified updates or service packs installed in order to get these updates.
Windows 7 Extended Security Updates
Recently, Microsoft announced that it is offering security updated beyond that 14 January 2020 date for users that want to continue running Windows 7, right up to January 2023.
This doesn't really apply to home users - it's aimed at businesses as it applies only to Windows 7 Professional and Windows 7 Enterprise versions.
On Microsoft's 365 blog, it explains that the extended updates will be "sold on a per-device basis and the price will increase each year".
The prices themselves haven't been announced, but it does provide a way for anyone running these versions (even home users) to continue to safely run Windows 7 for another 4.5 years.
The blog goes on to explain that "Office 365 ProPlus will be supported on devices with active Windows 7 Extended Security Updates (ESU) through January 2023. This means that customers who purchase the Windows 7 ESU will be able to continue to run Office 365 ProPlus."
Again, that's much more likely to apply to businesses than home users.
If the prices prove too expensive for you, or you're running Windows 7 Home, then it's unwise to run Windows 7 beyond 14 January 2020 if your PC is connected to the internet. That said, you'll still be able to run antivirus software which will provide a layer of protection.