As others have said, the 8 is effectively a 7S, which in any other year would have garnered mostly positive reviews. But because the 8's are now second fiddle to the X, the natural thing to do is to question why someone would upgrade to the 8 instead of the X.
For those currently using a 7 or 7+, it likely is a stretch to think upgrading to the 8 makes sense. I've been using my 6S+ without feeling the need or want to upgrade to the 7, and the I'm sure I'm not alone in thinking that the obvious upgrade path is going to be the X. For early adopters, why would you not skip the 8 and go right to the X?