Why wouldn't you? "Inflation"!!!
For one, inflation is lower than your 3%, that's the number one reason...i'll humor you anyways.
Two, if you're married, the first $12,400 of your annual mortgage interest are not really deductible since you're giving up your standard deduction to do so.
For a 750k mortgage at 3.5% over 30 years the annual interest only drops to 12,400 dollars right around the 20 year mark (the same as the standard deduction). At 20 years you've paid $400k worth of interest, with 250k of that interest having absolutely zero impact on your tax savings since the fed was going to give you that anyways.
That's why
I understand that investing that money can net you a greater amount than paying the 3% annually. Once again, i said if you don't have the means to pay off early because then you obviously don't have the means to invest additional funds else where.
From an investment standpoint i can see the risk/reward. From just a living stand point the risk greatly out weighs the reward imo.