I was considering adding my own answer to What are the differences between 는 / 은 and 가 / 이?. The basic thrust of my answer was going to be that the two are actually logically quite different; 이/가 is mostly a question of indicating grammatical subject, while (ㄴ)은 is a kind of selector or indicator. In that sense, I was going to say that they shouldn't be too confusing.
However, if it is true that the two are so different, that raises the question : why can't the Topic Particle be attached to the Subject Particle, given that it can be attached to other particles:
저는 아침엔 바빠요 - I'm busy in the mornings.
이 버스는 종로론 안 가요 - this bus doesn't go to Chongno.
돈이 있으면은 사껬어요 - I'll buy it if I have the money.
It wouldn't seem illogical to be able to say '사과를은 먹고 싶어' or '그 사람인 파리에 갔어요', but I don't think those are possible. Why not? I am interested in reasons from evolution of the language as well as logical grammatical reasons.