Because the housing market would readjust to an affordable level due to the massive increase in the number of homes available for non-portfolio buyers to buy. Simples.
Would Absolutely Love It (said in a Kevin Keegan type voice) if budget measures forced landlords to wholesale ditch theiri 'Property Portfolio' so individual families can afford their own home. Ditch your Property Portfolio and get a proper pension like the rest of us instead of profiteering on...