TSN isn't as starved for content as you think. First of all MLB doesn't run at the same time as the NHL for the most part. They also have CFL and NFL rights which put up very good TV ratings.
Lastly, the Blue Jays have the national rights for Canada west of Eastern Ontario. They aren't going to give that to TSN, so its not like TSN will be showing Expos games nationally on all of their channels. However, they still would get pretty good ratings from Ottawa to the Maritimes but I am not sure that justifies building a stadium in Montreal.