I hire accountant to do my taxes and save thousands of dollars every year over what I'd pay if I used turbotax. That's why I recommend hiring one once you venture into high six figures and up.
Equities are currently overprice. I'm not saying don't buy them at all, but expect a correction in the near future.
If not holding a mortgage is a "bad idea," then by that logic you should immediately take one out if you don't already have one. Do you plan on taking out another mortgage once yours is paid off? Would you take out a mortgage on a home you own free and clear? Do you buy stocks on margin? Do you take out personal loans to put in the stock market? Do you have a home equity line of credit that you use to put in the market? Why or why not? I mean, if you're expecting 18% returns, there's a thousand ways you can borrow money cheaper than that.
Everyone's situation is different so making blanket statements isn't appropriate. I'm not saying that they should immediately pay off the mortgage, I'm disagreeing that paying off the mortgage is a "always a bad idea."
Anyways, it's all personal preference. Some people are only happy going after the best returns, some want more security with less risk instead. No choice is better than the other, it's what makes you, personally, feel good. It also changes with age and life situations too. My parents are elderly now and now they feel really good with low risk low returns investments. This was not always the case. My investments increased in risk after I got married because a single income can cover all of our expenses but we have two, there's so much more room for breathing now.
Personal finance is not just numbers on paper and bigger numbers are better. It's personal, hence the name.
You never answered why this person should retire in 30 years. If they are 45 why wait until 75 to retire, especially with such a big windfall? If they are 22 with no savings, sure, $1M might not be enough to retire yet but waiting 30 years is super overkill with such a big windfall.
We know nothing about this person's finances, or life, so the sort of advise you gave is inappropriate to offer.
Just also know that people much smarter than you or I are choosing to put vast amounts of money into mortgage lending rather than the market. If the stock market was such a superior investment, why would any bank choose to hold a mortgage?
Fact is, you have to factor the risk into the equation when comparing mortgage vs market. Market volitility decreases market return and the extreme burden of losing your personal residence increases the mortgage rate side of the calculation. Also, of course, there is the negative affect on your credit and possible bankruptcy to factor into the risk equation (which also carries some upside in that if you don't pay of your mortgage, and lose it all while upside down on the mortgage, you have transferred some of the loss to the bank.)
And of course like the other poster said, "not paying off early" is the exact same thing as taking out another mortgage the day you pay it off, if you have the means to do so.
If you are really "doing the math" then you have to do all the math.