The Cupertino company's upcoming iOS 11 software update will not be available for the iPhone 5 and iPhone 5C or lower, meaning many will likely need to fork out for a newer model in order to see watchOS 4 appear on their smartwatch screens.
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The Watch has always required tethering to an iPhone, despite some standalone functions, which will result in the smartphone requiring the latest software in order to take advantage. This follows a previous trend in watchOS updates, also, with iOS 9 needed for watchOS 2 and iOS 10 for watchOS 3.
Unfortunately, since one of Apple's priorities is shifting users along to its latest software, this isn't expected to change when watchOS 4 rolls out, likely alongside the Apple Watch Series 3, later this fall. However, this does represent the first time Apple has changed the iPhone model requirement.
Of course, another group potentially affected by this scenario would be those looking to pick up an Apple Watch for the first time, with all devices running the new software out of the box once it releases later this fall/autumn.
But while it would seem gloomy for old iPhone adopters, this could change if the Apple Watch ditches all requirements and goes it alone over Wi-Fi/cellular means through its newest iteration.
Until that becomes a possibility, though, you may have to consider paying up for a new iPhone. Sorry.