Pensions should be funded by NI contributions. They're sustainable if NI is properly ringfenced and invested but UK governments run a Ponzi scheme, they spend your NI and pass the burden to future generations.
It sort of works if the population and economy keep growing. It fails when birthrate falls and the economy stagnates.
It's not pensioner's fault that government failed to manage the funding crisis that it's irresponsible policy led to.