Tax is (in a transparent govt.) used mainly to finance public goods and combat externalities. Its not an investment (well, not directly!).
Imo, its the responsibility of the individual to ensure their pension rather than making a fuss over the retirement age. Again, the power of compound interest is there for the lazy people. $100 saved over 40 years can be a lot of money in a compound rate of 8-10% (more than he would, in a normal circumstance, require in his old age)
To finance public goods and combat externalities, welfare maximisation, that is the aim. And retired people who do not have a pension schemes
are an externality for the society. They shoot up poverty rates, decrease consumption, and hence investment, economic growth... the usual trend.
We shouldn't, however, forget the political and social aspect of not providing pension schemes to people. Like you made it clear above, there will no incentive to pay tax if people know it will not come to use in their old days. Any government has to be highly vigilant of his opposition party. They will strike where it is going to bang more. A 'wise' government that wishes to remain in power knows how to play with the psychology of its people.
It is the responsibility of people as much as that of the government, I don't deny. But it's also their responsibility as 'responsible' citizens not to commit crimes, frauds, corrupt, kill, etc. Bottom line:
it is their responsibility, but who cares?