Subject: Re: Portfolio for a 90 year old
The only issue I can think of is that TIPS throw off some phantom income each year until you dispose of them and begin some sort of immediate annuity. That means there will be a tax expense each year that has to be accounted for.
Another issue may be if the end date happens to coincide with an artificial zero interest rate period. In that case, an immediate annuity may pay less than expected, while inflation is still higher than the interest rate.

For the first problem, maybe you need some income during the intervening years anyway, so you might just include the TIPS coupon income when calculating how much that would be. If you want to defer it all, you could buy TIPS STRIPS. Same yield to maturity but no coupons along the way. I've heard they aren't too hard to buy.

For the second problem, that the future date that you buy your immediate annuity might not be a good one for the implied rates, that's true. The variation is normally within a range, so it's not an infinite risk. Best way to mitigate that is either pick multiple expiry dates and buy multiple annuities over time. (you probably want more than one to reduce counterparty risk anyway). It's less likely that the returns on offer will be abnormally low on all of those future dates. Another possible mitigation is to keep a bit of an eye on things and earlier if a good opportunity arises. Sell the TIPS rather than waiting for maturity on them. Not perfect solutions, but better than nothing. And probably still much better than taking full inflation risk with full lock-in on a deferred annuity.

Jim