It links to a broader problem around the social contract breaking down - trying to make changes is going to focus attention on the elephant in the room, which is wider social unrest if unpopular changes are made, even if they are necessary.
Using gateway benefits as a marker for eligibility sounds like a logical idea. But as PP have already pointed out, there are sizeable numbers of people who have never worked and will probably never work. This isn't debating the rights and wrongs of the state safety net - people who need help, including if that means they cannot work, should get help. But the volume of people not working is not sustainable.
Someone who has worked all their life, with earnings that take them just above the gateway benefit threshold, is likely to be understandably unhappy if they are then told that their SP is to be means tested. But equally a young person entering the workforce, with its now-inherent insecurity, eroded protections, and diminishing returns - you can understand why some of them think "why bother?".
I think the state pension is the canary in the coalmine for a much wider problem which needs root and branch change of our economic system including taxation and social benefits. People should not be penalised or financially dis-incentivised from working. Likewise, companies should not be able to set up elaborate corporate structures which result in them paying artificially low rates of corporation tax.