It makes absolutely no sense for Apple to maintain phones having the same form factor as the 8, 8 plus or anything older. The iPhone X is clearly the future, and continuing to sell these older models will require developers to continue to develop apps for both screen ratios and with/without accounting for the notch. Not to mention, developers have already had to account for the regular vs plus iphone screens, and now have the X and potentially X Plus in the future - that's 4 different aspect ratios and screen layouts to develop for. No way Apple lets that become the norm.
So, what happens if Apple gets rid of the 8 and SE? They suddenly have no phone under the $1000 price point - this just won't allow them to compete with any other manufacturer. So they'll either drop the price of next year's version of the X to $800 or less and introduce a plus model at this year's X price points, or they'll introduce a new smaller model with the X form factor. If they go with the latter, they could potentially have an all new lineup of phones ranging from $650/$700 to $1350 or so.