it doesn't cover you getting disabled so you can't work anymore at the age of 30 after working 8 years.

1600% higher return is great when you work from yours 20s to your 50s/60s and can essentially self insure yourself at that point with it, but as the person you are responding to you is (I believe) trying to say, that's not everyone.

Fair, but given that we're talking about a hypothetical government program, "index-based retirement fund" doesn't imply you're the only one contributing to said fund.

in the country I'm now in somewhere around 18% of my salary is basically put into my "pension" (basically equivalent to a 401k). around 6% from my salary, around 6% match from employer and around 6% for severance (that one gets when they leave said employer, but can stay in pension, and most reccomend it stay there unless you really need it to make ends meet as otherwise pulling out 1/3 of your pension contributions).

This is external to disability insurance that we and our employers also have to contribute to. So I'd agree to a large extent that the govt forcing employees and employers to contribute significant amounts to 401k like programs could be in the interest of everyone in the USA but one needs a disability insurance program on the side as well and that isn't particularly cheap.