This is a very interesting topic and discussion.
I hear what you're saying re: the stock market not being guaranteed, but I think that's a far too simplistic approach. You're completely ignoring opportunity cost of your capital and most importantly, you are taking a short term view.
Global stock markets have returned 9.6% annualized over the past 115 years (1900-2016). This is through 18 business cycles (as detailed by the NBER Business Cycle Dating Committee), two world wars, the great depression, the great recession, the cold war, 17 US Presidents, terrorism, interest rates at 20%, interest rates at 2%....
Here's what $1M growing into $42B looks like by simply being invested in global equities. That is, the US, Canada, Europe, Japan, Australia and a composite of Emerging Markets (Russia, China, Brazil, India, South Africa, Eastern Europe, other Asia, etc etc):
Yes, it's not guaranteed as you say, and there are certainly drawdowns. But to me the biggest true risk with investing in global markets is a) your own behavioural and cognitive biases, and b) the way the system is designed that completely misaligns owners of capital (you) with managers of your capital (topic for another time).
More to the point, especially given that you can invest in this type of strategy for 0.25% or so these days, simply participating in global markets and adding when they drop will put you way ahead of paying off your 3% mortgage.