I expect that it's the long time horizon until Max-Hervé (or rather his estate) is actually paid off. Up until that point, barring any regulations which require the insurer to steadily accumulate reserves in response to the rising value of his policy, the insurer will keep operating - and profiting normally. It's entirely possible that this distant future cost isn't that much of an issue for most investors! And of course Aviva has 50 years to engage in legal wrangling as well. It would only take one amenable government in that period to turn the tables.

For Max-Hervé and his banks there is actually some rather significant risk. Their insurer could collapse due to some other cause, and policies might not be paid out. By keeping the policy and not settling, their taking out a position on Aviva's continued survival for 50 years.