You are clearly able to justify the expense to yourself but as you say, every individual is different. I use my iPhone for both business and pleasure, granted it’s not my primary computing device because it can’t run SolidWorks, Matlab, PDM or SAP. I’m under no illusions though that the iPhone 6S is no less capable than a brand new iPhone X or even the rumoured Plus for what I use it for. Taking photos of product testing, my kids, email and social media is the most taxing things I do and every iPhone does that. When I upgrade it’ll be to keep a device that is fast under the current version of iOS. I do think the prices have got out of hand, but the lack of current iPhones I am seeing suggests trends are beginning to change in my part of the world.
I’ve always been amazed how few Plus sizes iPhones I’ve observed over the last 3 and a half years so I expect an X Plus will be like hens teeth.