It is expected the X will outsell the 8. This comment has nothing to do with that, only your earlier comment about “fatigue“. I do not believe the current form factor will be retired for a few years.
If the X is outselling the 8 than lets examine the difference here. The only feature which the X has over the 8 which is user facing and a major selling point is the bezelless design and the oled screen. I held the X in hand the other day and someone coming from a Plus is downgrading the screen size for a higher quality smaller screen just so he can enjoy that new design and personally despite the X being more vibrant the 7 has the bigger screen which has a much bigger impact. So if the X is still outselling the Plus ,its just the design aspect of it and a better camera.
As far as the home button iPhones go, I believe they exist for 2 reasons. Supply problems and a test run of the X price point. I believe those supply problems will be ironed out by next year. If enough people buy the X (i.e. for instance 10 to 1 goes for the X vs the 8), they will discontinue the older design and go all in on the X. Also they are quite safe in doing this because Android is now charging the same price as the X for its flagships so its not like the customer is going to find a much better deal on the opposite side.
However as I said design fatigue will entice even Plus users to go for it and it will end up meeting Apple's set expectations to discontinue the older design. Heck, I dont like the smaller screen yet the design entices me