Not trying to derail this at all, just answering you.
Roof and upper deck were Leiweke's vision for an expanded BMO that could host outdoor hockey, Grey Cups and big TFC games. Like any business, revenues have to offset expenses and therefore the necessity of the groundshare. And they invested in that back up pitch, technology and a great grounds crew to ensure it had the best chance to succeed.
Basically, without groundshare, the politics and economics of expansion would have been difficult.