Alternatives.
The pension is means tested - discourages low earners from savings, makes those not getting it at all resent paying for it and look to reduce it (see this thread). High income people already pay 40% of it back through tax.
The pension age is raised - people have to work longer or save for the 4 year difference
The pensions value reduced- people can't live off it, so have to save to top it up or work longer
We tax high earners more - they are leaving in big numbers
We tax wealth through different land taxes and inheritance taxes - not popular with voters
We tax all earners more - has anyone got spare money, would you rather a compulsory private pension or tax and a future promise?
We increase productivity - how
We build retirement homes in rwanda (joke)
These all have issues. And realistically low paid workers are going to come out worse in any system.