The main reason I left academia is that I didn't think I had enough interesting things to say about the 20ish books I was an expert on to sustain me through a 40 year career. Basically, I just couldn't imagine giving the same lecture on Nietzsche until I was 65 (or 80, or whenever faculty will retire in the 2050/2060s).
I came to this conclusion fairly early on and it was reinforced constantly in graduate school. A professor I studied with, who was a wonderful teacher, told me he went back and reread the books for his introductory class every 5 years or so, "just to keep the subject fresh." After TA'ing his class (which was excellent) I had a feeling he hadn't refreshed his lectures in way more than a decade. He didn't need to — he knew what he thought about Marx and what he thought about Marx was interesting to freshmen. The idea of doing this myself, though, made me miserable.
The problem, of course, is that I hate lecturing and love reading. I never found teaching Plato very gratifying, but I often wish I had time to reread The Republic. (I've only done it once since graduating, sadly.) I am grateful that I knew myself well enough at 25 to realize that a life as a professor would make me miserable.
I rarely have any occasion to remember this, though, since I have been out of academia for so long. It's occurring to me now because I got an email that my dissertation adviser was speaking on a panel about the future of conservatism. (My dissertation adviser is, according to his new updated bio, a "leading public intellectual of the New Right." Insert puking emoji here.)
I was quite intrigued! The man I knew was hardly so political. Would he actually talk about politics as a representative of the New Right?? Or would he give the same talk about Tocqueville that I have heard him give a hundred times to every type of audience (grad students, freshmen, faculty, the general public)?
And it was the latter, of course. A talk remarkably similar to one I heard him give for the first time in a graduate seminar 15 years ago. He even opened with the same story about reading Democracy in America for the first time as an unemployed graduate student and realizing he would work on the book for the rest of his life. I have heard that story at least 20 times, and he is still telling it.
I remain a big fan of my adviser, notwithstanding his insane politics. When Josh and I were first married we had him over to dinner and it is likely to remain the best dinner party of my life. (There was no small talk, he did not even ask about our recent wedding. We talked about ideas from the first minute to the last.) And his talk about Tocqueville is, believe it or not, still interesting! I watched the whole panel when I definitely should have been doing something else.
I am not saying there is anything wrong with having a few good ideas and then refining and repeating them for 40 years. I'm just really happy that I don't have to.