My prognostication (for Debian and by inference Devuan):

With the glacial progress, missing drivers, and feature-incomplete status of Wayland, there is little to no chance it achieves default display manager status in Debian buster (freeze in 1.5 years, release mid-2019). Optimistically, it achieves default status in the following Debian release: bullseye (2021). And as we know, "default" does not mean retired or even obsolete. Considering the popularity of alternative window managers (like openbox and fluxbox) and no apparent heirs (save for i3/sway), X11 will remain the display manager of choice for these users. And with the difficulty Wayland has providing current hardware with video drivers, I wager it ultimately fails to provide adequate support for "older hardware". These shortfalls will prolong the death of X11 beyond buster's EOL (2024) well into the next decade.