You seem to think congress would borrow less if Social Security was investing elsewhere. They wouldn't. They would still borrow just from someone else.
If Social Security was privatized and completely out of the hands of the government, and if Social Security invested in the economy at large instead of US treasuries, the following would occur: Social Security would probably reap larger returns, but would also be more risky. Congress would still borrow and spend just as much as before, they'd just borrow from different lenders.
The problem isn't social security or even where social security invests. The problem isn't the Federal Reserve. The problem is Congress and a citizenry that is content to send shysters to Congress.
PS, You'd still be on the hook for disability and indigent care, so there would be a separate smaller tax. But if there were no Social Security, I believe it would be quite a bit bigger than it is now. More people would be in that category.
There is no such thing as a lockbox. The only thing more ridiculous than Al Gore claiming he would put the funds in a lock box was George Bush claiming he would do the same thing. Such a thing doesn't exist.
Oh, you could put dollar bills in a safe somewhere, but inflation would reduce their purchasing power.
You could buy commodities like gold, and put that in a safe. Of course Gold fell from $800 down to $100 in the 1980's. That's certainly not safe.
You can invest in the stock market, not safe. You can buy corporate bonds. But corporations spend the money as soon as they get it too. They don't just borrow money to sit on it. Again not safe.
The safest investment in the world is still considered to be U.S. treasuries. That's where Social Security invests.
A forced savings plan is not charity. Maybe it's not total freedom but it's not charity. And recognition that state responsibility for the indigent is a civil duty predates colonial times. So pick your poison... a higher tax for indigent care...or a forced savings plan that you benefit from and that keeps a lot of people from being on the indigent roles.
Right now, we pay 15% for SS. 20% is considered enough to cover a decent pension in every economy in the world. So why does our 15% return so little that everybody here needs to have an additional 401K/pension? That's because the government throws the money away. And they always will.
Charity is not a valid government function. If the society is charitable, government efforts aren't needed. And if society isn't charitable, then a representative government won't be either, except hypocritically.
Forced saving into SS when they intend to make it means tested is indeed forced charity. Forced saving into SS when so much of it goes into SSI is forced charity. Or simple wealth distribution.
The status quo may be as you say, but it's not moral.
Life is risky, and you can't change that. Rather than invest in the promises of men to give me back colored slips of paper in the future, I prefer to invest in things. Gold still buys a suit of clothes, as it did in the Roman era. I have some silver quarters from 1962. They would buy a gallon of gas back then, and treated as bullion are worth a bit more than a gallon of gas today. But treated as coin of the realm, won't even buy one Snickers bar. Over 40 years, that's about 6.58% over 40 years, not too shabby, but just keeping up with inflation, and I'd lose money if I sold & paid taxes.
Getting rid of SS might not make the politicians spend less, but it might let me (and other responsible folk) save more. It'd certainly help my kids and grandkids.
Getting rid of this regressive tax is reason enough.