Not crazy at all but definitely comes with major risks as many have pointed out.

A little different version is in my case I had cash from the sale of some family land and instead of paying off my 2% mortgage, I chose to put it all in the market.

Similar thought process - put into a broad US market ETF and let it earn 7/10% while I continue to pay off the 2% mortgage like normal.

Hopefully, it all works out - I guess for me the key is long term - I can pay the mortgage no matter what happens AND I do not need the money for 10+ years.