In Canada, most people lock into their mortgage rate for 3-5 years. After that, you've got to renegotiate a rate but you're also free to switch banks. It's like starting over again at whatever you currently owe. I locked into mine 4 and a half years ago, so renewal is coming up this summer. Meanwhile, my home's value has skyrocketed (thanks to an insane Toronto housing market).
What this all means is that in a few months, at my renewal date, the mortgage interests rates may be incredibly low (they're already at 2.7% today), my home's value is much higher than what I owe, and the stock market looks like it's going to be hitting the bottom around that time too.
So the question is... do I gamble on this? I could easily access hundreds of thousands of dollars in a low interest mortgage and drop it all on index funds. If it worked, 10 years later I could retire early. If it doesn't, I'm another god-knows-how-many years away from paying off the mortgage.
Mind you, my (sane, rational, smart-than-me) wife would never agree to any of this so it's only nice to think about.