Is it appropriate to reuse the same illustration in multiple papers?
No, there is no need to suggest removing the illustration. Why would you be concerned about it being reused? There is nothing wrong with re-using an illustration, especially if it "provides a concise metaphor of the framework the authors are working in".

Of course, the authors should state that the figure was previously published and refer to the original paper, as well as obtain permission from the journal it was published in. Given that it is their own paper in the same journal it will probably be no problem to arrange that.