I did a hypothetic exercise of calculating the internal rate of return (IRR) of a term plan held to 99 years old.
This plan is for a male non-smoker, 26 years old of age next birthday and sum assured of $120,000 for death, total and permanent disability (TPD) and critical illness (CI). Annual premiums is $891.60. Assuming the payout is made at the end of each policy year, the approximate IRR is as follows:
Beyond year 44, the numbers are an approximate as the premiums for the TPD should be deducted off (because it is no longer payable). The working for that is much more tedious so I have simply included it. The difference is not much ($32.49/year).
If a person has the discipline to actually pay the premiums all the way, a 99-year term insurance can be a useful tool to ensure a person has sufficient coverage (and if he wants critical illness coverage for life).
A whole life plan for the same person would cost from $2400 onwards (payable for 20 years) depending on which insurer you go to.