There's a second major Windows 10 update coming later in 2017, Microsoft has confirmed. The second update, code-named Redstone 3, will follow sometime after the putative April release of the Windows 10 Creators Update.
This new update was revealed at Microsoft Ignite in Australia via a "Windows 10 release cadence" slide, pictured below. The purple sections show the few months where an update is available via the Windows Insider programme, and then the plus sign indicates mainstream release. Teal indicates the period where enterprise customers "pilot" the new update, and dark blue is the "production" period where Microsoft provides active support. So, you can see that Microsoft plans to support two versions of Windows 10 concurrently before moving onto the next update.