I think to teach the books I teach, I need to have thought about them a long time and on my own terms. I think for students to study the books I teach is very useful, but the uses are basically impossible to predict...
I'm not saying there's an exact metric, but would you say that the work you do is useful? If a computer produced the same work instantly before your career started, so that you could build off of that point, or if it produced your work decades earlier so people knew about the results sooner, would that not have been more valuable?