From the Toronto airport, you also have a third option beyond Windsor & Niagara Falls:  the Blue Water bridge between Sarnia & Port Huron.  There's one VIA rail train per day from Toronto to Sarnia, and one Amtrak train per day from Port Huron to Chicago.  Both Sarnia & Port Huron have local bus service (albeit minimal), so getting from point to point within the cities would also be possible.  You might expect the Port Huron train to go to Detroit, but it doesn't;  it heads west initially.  If you really wanted to get to DTW, you'd have to change trains at Battle Creek.

The main problem with this plan (aside from coordinating all the connections) is that there's no pedestrian access to the Blue Water Bridge itself;  you'd probably need to hire a cab for that part.   However, [this guy](https://www.crazyguyonabike.com/doc/page/?o=tS&page_id=282403&v=1A) appears to have convinced the Bridge Authority to ferry him across from Canada to the US in one of their maintenance vehicles (in 2012), so maybe that's a possibility.