There is so much wrong and incorrect with comparing the mortgage situation today with pre 08/09 that it's like talking to a toddler about calculus, that's how utterly clueless those of you look when you argue that point, like clueless children. 

Buy this $100 doodad or have an extra 5 hours to spend with people really changes the calculus of these decisions.

Buy local, self manage, 1 business checking account, 1 umbrella policy, 1 LLC, 1 spreadsheet, Cozy, tenant selection calculus, M2M leases, trusted contractors.

This is basic math, not calculus.

Then I think only you can make that calculus 

That's a calculus that depends on supply and demand of your local market on one side and investment return on the other.You also lose tax deductible interest on the paydown, which shouldn't be a primary consideration, but should be considered none-the-less.Again, not a financial advisor, just a tax CPA

Once I figured out that my basic education in math (adding and subtracting) was more important than my college Calculus, (and accounting), I started smiling again.

I aced college level physics and chemistry and calculus 1 & 2 in High School.

@Matthew Paul But a "guru" of REI said wipe UP, not down, UP, so we must all do this, he's a "Guru" of REI, he fart's roses and cherry blossoms, defecates gold bars, in 3rd grade instructed his teachers on calculus, knew how to ride a bike before he ever saw one, and not only walks on water but waltz's on it....... 

Perhaps add that part to calculus.