There was a slight slump in rentals after the financial crisis, not before. So if anything it would depress the subsequent growth. In England the population grew over that period from about 51 million to 55 million. That's a 7% increase. Privately rented households, as per the link I posted above, rose by 63%. Hardly compelling.
>"In fact, from your own source, Figure 1 - the percent of rented properties is, well, nearly constant over the entire window (31% rented in 2007, 37% rented in 2017). And somewhere in there mortgage laws got significantly tighter - no need for pesky investors to be the cause of less mortgages."
Figure 1 shows private rentals increase from 13% in 2007 to 20% in 2017. Social rentals, which you are bizarrely conflating with private rentals, refers to social housing, and is exactly what I think should take the place of landlords. So its decline reinforces my point.