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 be insufficient over time?

For a linked annuity annuity, backed by conventional gilts over the mixed period, at 10 years, the probability of failure for MtM is 5.0%, For Off, the failure likelihood is higher at 7.7%. Other values can be seen on the chart.