We are both well aware that there is no risk-free investment. Governments bonds that are considered less risky that ETFs usually return 1-2% per year.
The ETF I'm talking about maps a market index covering a big chunk of the US market or several stable international markets. They are still a risk, your house is also an asset and is also a risk. I think if you only have your mortgage as an asset then probably buying some ETFs would help diversify your assets. But if you already have a lot ETFs then maybe would be better to finish up earlier the mortgage...