The Canadian version of Social Security is called Social Insurance, which is perhaps a better way of thinking about the issue. Social Security is insurance against being left absolutely destitute in the event total disaster occurs. Disaster can take many forms. You may think you have a secure pension, then discover that the plan has been mismanaged and all your contributions are gone and your contract with your employer is unenforceable. For example, when Detroit declared bankruptcy everybody who had spent their lives working for the city (teachers, police officers, municipal workers, and on and on) found their pensions were cut drastically. Their situation was made worse because the city was exempt from paying into social security, so retired city workers did not have that to fall back on. It has become quite common for pensions to disappear when employers declare bankruptcy. Another disaster is to become disabled at a young age, when you have not had enough years of work to save enough to retire on, Yet another could be to have a child who turns out to be severely disabled. No-one who has a child expects to be confronted with medical and care expenses totally hundreds of thousands of dollars a year, but this does happen to some unfortunate people.
If you think about social security as insurance, rather than as an investment, you can perhaps see the benefit a little more clearly. I do not expect to collect on the fire insurance I pay on my house. You might say I'm foolish to pay for insurance I will probably (hopefully) never collect. If I invested that money wisely, it might eventually grow to a larger sum than my home and all its contents would cost to replace. On the other hand, if my house burns down tomorrow, actuarial calculations of the hypothetical value of an investment thirty years from now will not help me.
If we were to make social security purely optional, we would have a situation very similar to allowing people the option of not paying for medical insurance, as was the case before the ACA and also the situation the Republican party is doing it's best to return us to. The issue here is that some (many) people will choose not to be insured, or not be able to afford insurance, yet many of those people will incur medical expenses. No-one plans to get cancer, or fall off a ladder, or (fill in the blank). (Similarly no-one chooses to have their employer go bankrupt and hand over the pension funds to creditors who never spent 5 minutes sweating on the assembly line.) This forces society to make a choice: step in and provide at least life-saving treatment at taxpayer expense, or allow large numbers of people to die of treatable conditions. As far as I can tell, the Libertarian position is "sucks to be them". That might make some sense from a purely selfish fiscal perspective. However, although I don't know how to express this as a financial balance sheet, I would suggest that a "society" that so lacks any sense of empathy that they would turn their back on people and refuse to acknowledge any value in them other than the size of their investment portfolio is so morally bankrupt that it is not a society at all. There is more to human societies than just a collection of dog-eat-dog winner-take-all investment strategies (where the best strategy of all is often to be born to rich parents).
Social security was created for a purpose, and that purpose was not the desire of some bureaucrat to invent a new way to pilfer money from people. It was the moral outrage of seeing people who had worked hard all their lives being left destitute in their old age, especially after the Great Depression wiped out so many businesses, banks, and personal savings.
One last point: is we were to make social security purely voluntary, we would have to ensure that every job paid enough to allow people to take care of their needs, the needs of any children they might have, and have enough left over to invest for retirement. How much would that be? It would depend on what rent, food, transportation etc cost so it would vary from place to place, but I imagine it could not be much less than $40-50,000 in most places. So all you libertarians who think you will save a few $$ by pulling out of contributing to social security, I hope you enjoy paying $20 (or more) for a burger at your local drive through.
Don