] But really, what do you suppose he was studying to be?

My opinion is that he was at Oxford because it was the thing to do and he could put off doing anything much for a while during his "studies." You might remember that Darcy and Wickham went to Oxford, and the former did not need to do anything in the line of work and the latter did not intend to.

John Thorpe seems to have had enough money to be a comparatively rich wastrel, which is what he was best suited for. I suppose that he might have gone into the Army, but there is no hint of that. Again, it was the thing to do, whether the education was put to use, or ever acquired. I believe that many of the upper classes considered an Oxford education worthwhile (I agree) whether or not it would be put to any practical use. Dorothy Sayers is eloquent upon the subject (See Gaudy Night.)