No company can indefinitely support older versions of their products forever because technology changes and usually results in improvements that can not be incorporated into those older versions of software and services.
Microsoft manages the support for their products through the Microsoft Support Lifecycle and right now it consists of two distinct periods of support called Mainstream and Extended Support.
Mainstream Support covers the first five years of a products existence and Extended Support covers the second five years to close out support at the 10 year mark. Occasionally support is extended beyond that 10 year cycle but that is the exception rather than the norm. For instance the very popular Windows XP started under Mainstream Support on 31 December 2001 and was covered by Extended Support until 08 April 2014 - a total of 12 years, 3 months and 9 days.
Service Packs for Microsoft products are typically supported for 24 months after the following Service Pack is released or when the products Extended Support lifecycle is over whichever happens first.
Now, under Windows as a Service (WaaS), there are no more Service Packs being released for Microsoft products, including the latest version of Windows (Home, Pro, Education and Enterprise), which means the end of lifecycle support is 10 years after the products release. In the case of Windows 10 that means October 2025 is when Extended Support will end with Mainstream Support closing out five years earlier in October of 2020. The one exception to this is Windows 10 Enterprise in the Long Term Servicing Branch (LTSB) which was designated on 02 August 2016. That means Mainstream Support ends one year later than the other versions of Windows 10.
Windows 10 is updated on a monthly basis through cumulative and security updates plus it is scheduled to receive one to two major updates each year. Those updates, either the monthly ones or the semi-annual major updates, do not change the support lifecycle for Windows 10 like Service Packs did under previous versions of Windows.
If Windows 10 is truly the last version of Windows that will ever be released we may see a change down the road with the layout of Mainstream and Extended Support and their expiration dates so that Windows 10 continues to be supported past the first 10 years. For now though it appears Microsoft is still approaching Windows 10 with the same two periods of support as they did with older versions of Windows.
You can research the lifecycle data on the Microsoft products you use at the Microsoft Support Lifecycle portal. Of course, cloud based services are changing support lifecycles so policies for Microsoft services like Office 365, Microsoft Azure and other online services are here with a specific Azure related support FAQ here.
Looking for an awesome, no-nonsense technical conference for IT Pros, Devs, and Devops? Check out IT/Dev Connections!