Its pretty funny stuff in this thread.
My one point is that it is easy to confuse a data based strategy with the statistics of probability. Morgage keeping strategies are built on a back testing of historical data models. They are designed to help us plan, but are not a pure system of mathmatic probabilities.
We are used to things like blackjack strategy win rates or poker strategies, where the odds are known, with possible outcomes,as finite and measurable. It is easy to think that stock market win rates can be similarly determined, but they are different.
The enthusiasm for a good idea, (leverage) is great, but dont confuse the useful model of a complex system with statistical probabilities. The models of past data generate convincing statistics, but all are backwards looking. These models are not necessarily predictive of future returns. They are useful, but I see many posts in this forum that seem to think that 30 year positive returns are guaranteed just because they have occured for all of the prior 30 year periods for which data is available.
Calling the best predictor we have sure fire 'math' is exaggerating the predictive usefulness of these studies. Sure, they are designed to help us make the best guess possible, and perhaps we structure our plan accordingly.
IMHO, this is not a good fact base from which to call people flat out wrong for not fully trusting these models and accordingly calling them stupid to not invest 100% is US stocks. It is exactly the reason we have prospectuses with all the language many ignore.
Keeping a mortgage is a good bet, but it is not a bet that 'cant lose.' (as often implied and flat out said by b42). No analysis of the past eliminates a poor result as a possible outcome. I dont care how loud someone like b42 yells, he is wrong when he guarantees future outcomes or calls paying down debt risky to FIRE. People who want less debt are fine (as are people who want to take on the risk of leverage and reap the rewards should stocks perform as expected).
The future is unknown and only a fool says that an investment strategy is fool proof because of the past, whether well researched or not. That is why we diversify. That said, the debates educate, so i guess i am fine with it all.