Why would paying off your primary residence give you more peace of mind than having $700k in liquid investments? You can’t use your paid off house to buy food or pay medical bills.
If your primary residence is mortgaged you're paying interest that compounds to a lot of money diminishing your $700K sitting under the mattress. Pay off the house and save + invest the interest you'd pay each month. That will afford a lot of food and doctor's visits later.
Who said anything about putting $700k under your mattress? Which is going to generate more income, investing the entire $700k or investing your mortgage interest?