Population growth isn't inherently bad, the GTA has always absorbed huge numbers of immigrants (in fact, its growth rate today is lower than it was 20 years ago)  but it's only in the last decade that housing has gotten out of control. 

Bear in mind that immigration is also a national issue - there are all sorts of unintended economic consequences you risk by cutting back on it, many of which would not be felt in the GTA. Winnipeg, for example, is experiencing its first boom in decades, and that's largely driven by immigration. Should that city be kneecapped because of out of control real estate speculation in the GTA?  It's far too blunt a tool for the issue being faced.