Oppose on principle. Its not something the government should decide.
Isn't it though? Assuming we don't want to let people go hungry, the state will have to provide a certain amount of care to ensure a proper living standard. And if people are aware that this will be provided, doesn't that give them an incentive to spend wastefully before retirement?
Seems like the debate on health insurance. As long as we aren't willing to let people die in the streets, it's certainly society's (i.e. the state's) business that people are obligated to have a minimum level of health protection.
Forced savings is a misfit. You either confiscate income as taxes or let people keep it. Letting the government interfere with how you use income that has already been taxed is a slippery slope where government gets increasing control over the citizens life choices.
I agree there needs to be a minimum standard of living, but government will simply have to pay a meagre "survival pension". Its just not the government's business how you should spend your income after having paid taxes. Lets say you decide you want to live your life in a way that makes it extremely unlikely that you will ever get old, doing lots of extreme sports and other highly dangerous activities. Should the government then be allowed to "confiscate" part of your income to a purpose you knew would never be relevant for you? That would be an unacceptable intrusion in personal freedom in my view.