I agree that such tax reform (and other regulatory measures) is really needed.
But, if the units are purpose built for affordable housing (as proposed federally in https://liberal.ca/housing-plan/ , for instance), this should at least not fall into the investor problem, no?
Ah, true. Reading https://liberal.ca/cstrong/build/ I don't see anything that says these affordable units will be kept off the market, or that ensures they will be rented at affordable rates.
I also think land taxes seem promising, and taxes on uninhabited excess square footage, that are earmarked exclusively for building high quality public housing.