Emathias, is the line of which you speak essentially the 1968 Monroe circulator?
I am with you on putting some variant of that where there is already existant intensity. Not to say a circulator such as circle line would not be useful as well. I think it would be. It is as you say a matter of priorities.
Ideally, i'd have such a variant of the monroe circulator, some variant of the circle line, and an outer loop maybe along western.
I really think if chicago's L system transitioned to a more dense graph from the relatively sparse graph it is now that ridership would would transition from under performing on a per mile basis to over performing on a per mile basis