Some people might value having the 7S over the 8. Obviously, some people will buy it. But the question of why so many people will flock to the 8 is answered by the fact that the 8 will be sporting something new, which is going to be priced and advertised as "the best". The 7S, just based on how Apple does the pricing, will be the inferior product for most people (unless you need Touch ID).
When all is said and done, I will be expecting the 8 to outsell the 7 and 7S combined.
Yes it will be something new for Apple. But everything the new iphone can do has been out on the Galaxy S8 for 6 months. And an unlocked S8 is $575 in Amazon right now. Used to be the iPhone was the cream of the crop and only it had all these great features. Other phones have caught up and are offering the same features as the IPhone. The one thing that keeps me on iOS is iMessage. It is flawless!!! If it weren't for that I would switch immediately to save some money.