If a friend gave you £1,000 each year for 20 years to look after for their children and invest on their behalf for the next 50 years, would you do it (unpaid, of course)? Broadly this is what pension scheme trustees are required to do, ie, honour a very long term promise made by an employer to its current and former employees.

That promise has to survive changes to, and the competing needs of, the trustees, the employer, the government, the law and tax rules. The long-term security of the funds is subject to the investment decisions made by the trustees and the economy as a whole.