These days most people are pretty rabidly pro-heritage preservation, and the laws are decent. The City even dictates acceptable paint colours in some areas. All of this a result of the outrage following the construction of Atlantic Place - that was the point when we changed from wanting to modernize and not be a laughing stock to industrial mainland towns, to wanting to preserve what we had. Dorothy Wyatt and Shannie Duff (former Mayors/councillors) longed to see it come down before they die
These days, anything that gets destroyed generally follows the same process - owner (almost always an absentee landlord in Montreal) neglects it, leaving it empty for YEARS, until its condition is so bad it "cannot be saved due to the high cost" (their insistence, not mine). Then its torn down, goes to some developer. If the City flexes its muscle at all, it's only to require off-street parking, limit the height, and insist on some generally hideous faux heritage elements.
It's not all dying, though, of course. Can't stress enough that the majority want to preserve it. Every year there are numerous businesses and private homeowners who receive Southcott Awards for restoring their buildings in a historically accurate way.