“Imagine if all your SS payroll taxes had been invested in an index fund.”
The operative words are “YOUR SS payroll taxes”.
They aren’t “YOURS” it’s tax collected by the government, so as soon as it left your paycheck, it ceased to be “yours”.
Don’t shoot the messenger. You should be angry about it, but you also need to understand what the premise of your question is fundamentally incorrect.
You are essentially asking “What if I didn’t have to pay taxes?” which would be an unquestionable benefit to every working American, but is a pointless exercise in futility.
Actually it is not a tax it is a contribution. FICA =Federal Insurance Contribution Act.