Most attempts to subsidise mortgages make existing property and landowners richer without broadening the number of people that are able to buy. Sometimes they backfire by making deposit requirements bigger. See Help to Buy etc in the UK.
Don't get me wrong, that would be great, but you didn't outline a mechanism by which MIRAS increases social housing provision. I don't think there is one. Arguably the causality runs the other way as tax giveaways to homeowners reduce gov't revenue and spending.
So basically the same as the UK with no subsidy?
Most attempts to subsidise mortgages make existing property and landowners richer without broadening the number of people that are able to buy. Sometimes they backfire by making deposit requirements bigger. See Help to Buy etc in the UK.