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This all would be true, except for the bit that you can't calculate. Which is the likelihood of the annuity continuing to payout.

You'll notice I said continuing to pay out.

My grandmother had an annuity before she died. About 10 years before she died, they sent a letter basically saying that they weren't going to pay out anymore. She worked as an accountant all her life (even before the Rosey the Rivetter years) and kept fucking meticulous records. Honestly, the record keeping she had was impressive enough that it deserves its own post but that is neither here nor there.

Anyways, she still had the paperwork. It had no end clause except death. It was an annuity.

It wasn't even that much. A couple of hundred a month. They just decided to stop paying because they figured it was highly unlikely that she would fight it.

She was still with it mentally mostly, but my father was doing most of the finances for her by that point because she was in her early 90's already. And they were right. Fighting it would have taken too much effort.