Call it a benefit, call it an entitlement, an insurance, a pension - I don't care. But whether or not there is a 'pot' (and the fact that there isn't an individual account or personal fund is always expressed in this way) is neither here nor there. The rhetoric feels like one of those things that is planted so that if we hear it often enough we'll believe it to be true.
People were told that if they paid in for X years they would get Y back at Z age, and have planned accordingly. The age has already shifted, causing a lot of hardship. Means-testing the pension would be political suicide, because it would be ridiculously unfair. Those who haven't paid in would still have to be provided for, so what would those who have paid be told they had paid for? We know we've paid for the generation ahead of us, but that was on the understanding that subsequent ones would pay for us.
Yes, there have been demographic shifts, but governments through the years were aware of those as they happened. It was their role to ensure that this was managed, and if they failed at that it is now their role to manage things to honour the promises of their predecessors. Not to do so would be shameful.
It may be that changes have to be made for the future, but people who are now paying NI would have to be able to claim based on their payments to date, and given plenty of time to adjust to whatever scheme replaced the current one. My work pension changed from Final Salary to Career Average, but was phased in so that those within X years of retirement stayed on the old scheme, current employees moved to a hybrid version and new starters went onto the new one. Something like that might happen, but as it would mean that it came to pass with a different government in charge I think it's possibly unlikely. Who would want the flak without the benefits? It would be a very brave move - it's possible, though.