Wayland is now the future of Unix graphics and GUIs
The big Unix graphics news of the past week is that Ubuntu threw in the towel on their Unity GUI and with it their Mir display server (see the Ars story for more analysis). I say 'Unix' instead of 'Linux' here because I think this is going to have consequences well beyond Linux.
While there was a three-way fight for the future between Wayland, Ubuntu's Mir, and the default of X, it was reasonably likely that support for X was going to remain active in things like Firefox, KDE, and even Gnome. As a practical matter, Mir and Wayland were both going to support X programs, so if you targeted X (possibly as well as Wayland and/or Mir) you could run on everything and people would not be yelling at you and so on. But, well, there isn't a three-way fight any more. There is only X and Wayland now, and that makes Wayland the path forward by default. With only one path forward, the pressure for applications and GUI environments to remain backwards compatible to X is going to be (much) lower. And we already know how the Gnome people feel about major breaking changes; as Gnome 3 taught us, the Gnome developers are perfectly fine with them if they think the gain is reasonable.
In short: running exclusively on Wayland is the future of Gnome and Gnome-based programs, which includes Firefox; I suspect that it's also the future of KDE. It's not an immediate future, but in five years I suspect that it will be at least looming if not arriving. At that point, anyone who is not running Wayland will not be getting modern desktop software and programs and sooner or later won't be getting browser security fixes for what they currently have.
People run desktop software on more Unixes than just Linux. With Gnome and important desktop apps moving to Wayland, those Unixes face a real problem; they can live with old apps, or they can move to Wayland too. FreeBSD is apparently working seriously on Wayland support (cf), and at one point a Dragonfly BSD developer had Wayland running there. OpenBSD? Don't hold your breath. Solaris? That's up to Oracle these days but I don't really expect it; it would be a lot of work and I can't imagine that Oracle has many customers who will pay for it. Illumos? Probably not unless someone gets very energetic.
With that said, old X programs and environments are not going to
suddenly go away. Fvwm will be there for years
or decades to come, for example, as will
xterm and any number of
other current X programs and window managers. But people who are
stuck in X will also be increasingly stuck in the past, unable to
run current versions of more and more programs.
(For some people, this will be just fine. We're probably going to see a fairly strong sorting function among the free Unixes for what sort of person winds up where, which is going to make cultural issues even more fun than usual.)
PS: Some people may sneer at 'desktop software and programs', but this category includes quite a lot of things that are attractive but by and large desktop agnostic, like photography programs, Twitter clients, and syndication feed readers. Most modern graphical programs on Unix are built on top of some mid-level toolkit like GTK+ or QT, not on basic X stuff, because those mid-level toolkits make it so much faster and easier to put together GUIs. If and when those toolkits become Wayland-only and the latest versions of the programs move to depend on recent versions of the toolkits, the programs become Wayland-only too.