The question is - what else would you do with the money?

I wouldn't stick money in a low-interest mortgage if I was willing to tolerate some risk for a better return. Paying down a mortgage @ 3.5% is not a 'great' investment as you've pointed out, but the key is that it's risk free AND a better return than other no-risk investments (cash).

You may want a no-risk investment if you plan to use the money in the short-term after liquidation, say for another home purchase/deposit.