Never heard of it, so just googled, but it's not obvious why these would be more preferable to annuities. As you get older above about 75, people tend to spend a lot less in general (assuming they don't need care support), so getting more money when you're unlikely to spend it anyway seems pointless.
I'd speculate that the regular payments on annuities would therefore be higher than tontines because insurance companies would be able to factor in average life expectancy to the pricing more easily, rather than having to budget for possibly all the money continuing to be paid out to just one remaining person who happened to beat the odds and live well beyond 100 instead of the average around mid-80s.
I would change it to "the expected natural life of the author" which is their birthday + 85 years. Regardless of whether they actually are still alive past that date.
This can't work: life expectancy (of humans) can differ more than 30 years around the world, plus there are significant differences between men and women, and corporations - which often own copyright on work for hire - can live in perpetuity.