Albeit I’m sure that a lot of X11 window managers will continue running even if its code-base will be untouched for many years, every major distro is going to default its graphical stack to Wayland. Eventually, some programs will only be compiled with Wayland support, notably commercial pre-compiled applications. At that point (maybe 5-10 years from now), I would expect that nobody will maintain packages that involves X11 window managers. Regardless, XMonad still be maintained for X11 while its Wayland version is developed.
I’m not sure about the abstraction layer, while it’s a great idea to have some kind of modularity, almost every graphical stack developed nowadays is based on Wayland and/or written in C++, which is a little bit painful to write integrations in Haskell. Workstation users are not that predominant today, so I don’t expect anybody developing another graphical stack, in particular for desktop/workstation usage, and I also cannot see XMonad being predominant at some environments like automobile interfaces or small devices, even if it can do it with some kiosk mode.
What it can relieve some migration pain is great docs, in particular about switching from X11 to Wayland configuration. I’m pretty sure that newcomers didn’t write a thousand-line config their first time with XMonad, so it’s unrealistic to think about changing only a few lines of code and run XMonad in Wayland seamless when actually there are no code written.