Taxes are an issue for people that aren't already rich enough to not care and the people actually hoarding residences and leaving them empty are in that category.
It seems like a lot of people have a very hard time understanding the governments division of powers and I think it's a very big problem because it gives the provinces a free pass for many things as people blame the federal government without realising that they don't have the means to solve these issues...
Cities aren't even political entities from a constitutional perspective, they're fully dependent of the provincial governments and are called their pets, so guess who could force them to increase density? Not Trudeau!