There is only one reason not to. Next year's iPhones will have specs similar to the X.
There might be one more reason to upgrade but this is pure speculation on my part : I think the speaker sound got even better on the 8 than the 7 but they didn't push that selling point yet...but too true, the X sure got the..err...SuperIdol factor ???
I'm still on iPhone 5 and not even pondering upgrade (maybe because I got it for tests and it so dame annoying to use...)