How will existing apps "scale" on iPhone X?
Right now most of the apps are made with Auto Layout, so they are not going to be "scaled" but rather position of the items may be different. The biggest problem is to handle the top area of the display. I checked some of the apps in the iPhoneX Simulator and apps that display status bar do not have any problems. With status bar hidden and without navigation bar there could be problems. Here's the Hero Sample app from YiLun Zhao :



As you can see, Tab bar is "stretched" downward, but top scrolled area is displayed rather strange. For example section separators are displayed under the "status area" (bottom of it), but table content is clearly visible underneath (side by side with black camera area).

I checked some other apps and there are no major problems with the UI.

So the answer to your question is - it depends how developers made their apps. If they've used Auto Layout there shouldn't be a problem.

P.S. even though the display is bigger than iPhones Plus it has "compact" horizontal traits in landscape -> there won't be split view. Also pages loaded in the landscape view do not occupy the whole screen width (there are white stripes on the both sides. Screenshot:



NOTE: This is still Xcode Beta, so maybe things will change when iPhone X is released.