You have an Apple Watch and are looking forward to the watchOS 6 update? Then you should have the right iPhone for Apple’s smartwatch.
As we wrote here many Apple devices fall out of software support with iOS 13. This is also important for Apple Watch users: Apple is so fair to support Series 1 to 4, but under one condition: the iPhone has to run iOS 13. So if you run your Apple Watch with an iPhone 5s or 6, you can’t upgrade your Apple Watch to watchOS 6.
So you are forced to buy a new iPhone or not to update. The unanswered question is whether an Apple Watch updated to watchOS 6 still runs on an iPhone with iOS 12 or whether there is no way back.