The endowment financial status is only assessable at the end of the 15 years, when there is just one cashflow to be paid out then. How likely is it that the annuity fund will become insufficient over time?

For a linked annuity, backed by conventional gilts, at 10 years, the probability of failure for MtM is 10.2%. For Off, however, the failure likelihood is zero. Other values can be seen on the chart.