No. I think iPhone 8 sales will actually help the X tremendously. Most customers will definitely be thinking “why get a phone when it’ll be outdated in weeks when the X is out?” and “I’ll just need to pay a bit more to get the X, so why the 8?”
I don't think that will be the logic by customers. Because the iPhone 8 has similar internals as the iPhone X, which wouldn't make it outdated. And if you're referring to the design of the iPhone 8 being outdated, that would be subjective based on one's appreciations. Not to mention, two major advantages with the iPhone X is the OLED display and face ID. Most want to experience Face ID, but it doesn't necessarily justify the price increase based on what they want from the newest iPhone.