But it's not wrong, in expectation given long term past trends. It's just leveraged investment using a vehicle for leverage that has some tax and refinancing ad

But it's not wrong, in expectation given long term past trends. It's just leveraged investment using a vehicle for leverage that has some tax and refinancing advantages that favor the borrower, not to mention the liquidity advantages. There are very rational reasons to do this. For any investor, there is a point in the mortgage interest rate vs risk-adjusted returns space at which investing is better. That point may differ, of course.