Of course the exception to this are old-school rock-stars (and the new-school rock-stars: DJ's & Producers). Not jealous of their life expectancy though.
A lot of creative people I know go through cycles and do just fine.
I’m on the extreme end of that due to being bipolar. I have days of extreme productivity and days of none. Likewise with creative ability. They don’t always line up all the time. I can be creative and depressed, or hypomanic but unfocused.
Even with medication, routine does not (and never will) exist for me. Learning to accept chaos and work with it instead of against it was the best thing I ever did for myself.
If I can be creative, I ride it until I need to pull back and let things settle. If I’m not, I work on more boring things like refactoring, testing, and documentation. If I can’t focus at all, I take time off to reduce stress until I recover.
You're absolutely right about cycles and all that. That's how I experience it as well.