Plateauing after 5 years? That seems quite early. Maybe something >15 depending on ones capacity to go further and learn more. There is more stuff out there than one could learn in a lifetime. Many decades of learning. And if one does not know what to learn next, just learn a new language and see how concepts one already knows apply there. Or pick up a book of the masters of our craft and work through it. Like for example one could ask oneself: Have I really worked through all of SICP (or insert other great book here)? If not, maybe there is lots of stuff in there to learn.

Maybe one plateaus after 5y of mainstream every-noun-a-class and endless-design-patterns-forced-in kind of stuff. I guess I would, if I did not look for more elsewhere. I think such kind of job is also why there is a disillusionment. One suddenly realizes, that at the job one might never apply all the cool things one knows. Then it is up to oneself to either find interesting side-projects, or deal with it in some other way, or quit.