I think my advice to my spouse, if she were to reach 90 without me, would be to take as much risk off the table as she can and just enjoy her time.

Unfortunately it isn't so simple in most cases. First off, I think you are referring to "risk" here as "possibility of loss due to investments declining". If that's the case, when you are 90, the decisions are difficult. Because then you are 90, you are usually close to death, maybe a year, maybe 5 years, maybe even 10+ years, but not much more than that. That means that you are close to the point of "capital gains step up" (which occurs AT death, in the USA). So getting back to "risk" ....

It could require selling up to $1M or more of assets to yield an after tax $700k to fund the annuity.

What was the risk before selling? Perhaps that the investments decline by 30% or so? And sure it is possible that it would happen, but it is also possible that those investments recover some of the lost 30%, even if you only have 1, 5, or 10 years left for them to do so. BUT, when you sell that $1M, and pay the $300k in taxes, that money, that 30% is lost forever ... it is never coming back. So in essence you trade a potential risk (investment decline) for a definitely realized risk (taxes). As far as managing investments, in general, it's probably best to have arranged (from age 50, 60, 70, etc) at least the bulk of your investments to require little management. That's very important because as we get older, we definitely decline, our brains decline somewhat in most cases, and managing investments gets more and more difficult with time.

That said, I think annuitization of some money is often a useful tool for people with longevity in their family. And if you do indeed have a good longevity probability then the planning for annuitization should begin long before age 90. I wonder if there are insurance instruments out there that have a smaller principal payment up front, and begin their annuitization at age 80 or even 90? I just checked one of those online calculators (schwab.com - Fixed income annuity calculator) and it says that if you are 60 today and invest a one-time $100,000 in an annuity, you will receive about $3600 a month beginning at age 80. So $11k a month would cost about $305,000 at age 60. And now I plugged in 90, and $100,000 at age 60 turns into about $10,400
a month beginning at age 90. That's pretty good longevity insurance!