"house prices would not be increasing in areas of population decline" -> the causation could be running backwards: a family of 2 (maybe 2 retired people with accumulated wealth) can outcompete with a young family of 4 for the same unit, that drives prices up and population down. I'm not saying this is the case, but decreasing population not being associated with decreasing housing prices does not mean that increasing housing prices is not caused by housing scarcity
When money was free and restrictions were few, non-institutional investors seem to have capitalized aggressively on this, because the price of the property increased dramatically and also it was a near-certainty that there would be a constant stream of inflated rent a landlord could charge.
I'm happy about those tax disincentives, but they do strike me as playing around the margins; in normal economic circumstances, they should play around the margins, but that particular aspect of the economy is so embarrassingly tilted that it's really not funny anymore.
The result is that in the last couple of years landlords have gotten rid of many rental units. This primarily affected smaller apartments, and surprise surprise, those prices have remained flat while the rest has increased.
On the other hand, sucks if you for some reason need to rent, because now there's just a few rental units out there and they're now expensive.