SBF should never be free in his mortal life. How many people would happily take 10 years prison in exchange for $1M, much less $10M, $100M or $1B? The total societal harm, and lives damaged has to be taken into account. He did more damage then a meth-head robbing a convenience store for $300 at gun point. It's not about revenge, or punishment, or rehabilitation, it's the principle of the thing. It's to prevent the next guy from trying the same thing.
You're also assuming that he hasn't locked away some of that crypto in hidden wallets, or in overseas bank accounts. People have a hard time wrapping their minds around the scale of this kind of fraud. When you grift billions of dollars, squirreling away 10's of millions is like pocket change, and would be nothing to hide. Ask yourself how did he afford his high price lawyers? If we did what you suggest, he would most likely just run another scam of some sort, or he could 100% leave the country and just live a retirement you and I could only dream of with his ill gotten gains. A guy who gave 100's of millions to politicians, (who by the way haven't given that money back), would never live a life of poverty for penance. Hell, the guy could probably sell SBF t-shirts or golden shoes, and make millions at this point.