X has long had critics from Unix fans. There’s an entire chapter devoted to X in The UNIX Hater’s Handbook, and the author of that chapter, Don Hopkins, is a major advocate of Sun NeWS. On the Unix side of things, alternatives to X included Sun NeWS and NeXT’s desktop, which are both Display PostScript-based systems. macOS uses Quartz, which is similar to Display Postscript but uses PDF-based objects instead of interpreting PostScript code for rendering graphics. There are also 8 1/2 and its successor rio from Plan 9, which also addressed issues that the Plan 9 developers had with X.

Of course, X won out over all these systems; while X has been MIT-licensed since its early days, none of X’s competitors were open source at the time of their release. NeWS, NeXT’s display system, and Quartz remain closed source, and while Plan 9 was released under an OSI-compatible license in 2002, it wasn’t GPL-compatible; this was rectified in 2014 when Plan 9 was GPL’d, and the licensing became even more liberal in 2021 when it became MIT-licensed.