I suppose I could be ok doing the same thing over and over. It's a rather easy situation to fall into in illustration. Art descriptions may get repetitive, you might get pigeonholed into doing certain types of illustration, and so on. Most folk didn't get into illustration to do the same thing over and over, however. A certain desire to grow, learn and evolve exists in probably every artist. But one thing can easily get in the way of all this:
Not having the freedom to fail.
Illustrators, like most folks, do not have the freedom to fail in their jobs. When you try something new--some new medium, some new approach, manner of composing an image, very different color scheme, and so on, you risk LOLcat levels of fail.
Failing as a professional by trying things out is a bad idea--you can lose a client in a heartbeat by pulling these shenanigans. I should know, I made these same horrible lapses of judgment myself throughout the 90s in my first years as an illustrator. You don't head in *thinking* you're going to fail, or you wouldn't do it. But when you do, all you can do is facepalm and tell yourself you should've known better.
So, for the past 10 years or so I've tried to give myself outlets for fail. Whether that be something simple like figure drawing, or these museum studies, plein-air painting, or other art experiments you maybe never see here, I've made an effort to experiment only off-stage. By doing so, I've been able to practice new methods, refine certain techniques, explore new media, and so on. If I fail (and it happens), I can bury it outside public view. If it goes well, I share it and then roll it into my assignment-based work, where it improves my professional art. My clients, therefore, never have to worry that I'm going to suddenly pull some Jeckyl/Hyde routine on them. And, honestly, by regularly injecting my illustration work with the positive results of these experiments, it keeps illustration work fresh and exciting, and predictable for clients at the same time. That's important for them.
But, being able to fail spectacularly, in private, is important for me.