I disagree with the "Ah, but they've paid in their whole life" argument. The whole point of a tax and benefit system is that is that the tax should be progressive to match to ability to pay, and the benefit should be based on need.
Otherwise we could start arguing that people can get rebates for not using NHS services, or for sending their children to private school - after all, if they've paid into the system, why shouldn't they also get something out?
The big problem is that pension provision has never been properly budgeted for - successive governments have dipped into the pension pot, on the assumption that continuing population growth will ensure that the cost can be paid for out of current tax receipts. In reality, with an aging population, we are in a situation where current tax receipts can't cover the provision as adequately as expected. But because the older generation are more likely to vote, policy is skewed in their favour, so instead of making OAP benefits subject to the same assessment of need, they cut other benefits / services instead.
I don't buy the "it's too costly to administer" line, either. How is it that a fuel allowance is too costly to administer, but working out liability to repay child benefit isn't? Surely once people are earning an income in retirement, for most of them it'll be relatively fixed, so it only really needs to be assessed once?