Don't you see the problem here? Few politicians would dare campaign to reduce an established benefit. At what point would reason step in declare the scheme unsustainable? Probably when it's too late and all the productive talent has left the country.
It would be fine to have a constant increase at every election of it keeps up with inflation. I think you're a bit biased based on your feelings of mankind. It could go multiple ways but almost certainly it won't be predicted here.
It could have nothing to do with what one thinks about mankind. When people adopt a system (or a system is forced upon them), people will act rationally within that system.
