Another often overlooked factor in where to build the stadium would be the sale of the corporate suites. For modern pro sports this is a sizable stream of revenue. Someone correct if I am mistaken but it appears that the suites at RC for Argo games are empty. So although RC may not have great seating for the fans sitting in the seats the suites in RC are first class as will be in any new stadium built.
For corporations there are too many options to where they would acquire a suite for hosting clients or for employees. Summer has the Jays and TFC going head to head with the Argos in the downtown area.
Markham is home to several multi national corporations main Canadian HQ; IBM, Motorola, Toshiba, etc.
The sales of suites are crucial to the new stadiums. Winnipeg is sold out with a waiting list. In Regina the temp seating that was placed in the end zone also included suites which sell out for games. Montreal's renovations included upgraded suites which I believe are also sold out on a regular basis.
So even if the Argos built a stadium downtown they would probably not have as much success in regards with the sale of suites with competition from the other 4 MLSE and Rogers owned franchise.
In Markham they will be the only game in town and the corporations are right there for easy access for say a big friday night football game on TSN after work, Thursday night games in the summer and all the comforts needed for a weekend afternoon game as the weather begins to get cold in the fall