In the case of the X Display Server, it was the successor to the W Window System:

<https://en.wikipedia.org/wiki/W_Window_System>

That's in line with a number of sequential naming series, as in BCPL -> B -> C (programming languages), S -> R (statistics languages).

Arguably there are also IBM -> HAL and VMS -> WNT.

The iPhone case made sense. Apple had Mac OS X, which was the system after Mac OS 9. It had major architectural changes, so much so that every major release was always 10.N, where N was a number from 1 to 15, plus a minor version (10.16 became macOS 11). The iPhone X was released on the tenth anniversary of the iPhone.

In both cases the X stood for the roman numeral 10 and the correct pronunciations for the names were “Mac OS ten” and “iPhone ten”. In the iPhone’s case, they even skipped a version (there was never an iPhone 9).

> the X display server…

From which he apparently got an idea or two about a certain logo.

https://en.wikipedia.org/wiki/X_Window_System

What's wrong with the 7900 XTX? They also have their x3d processors