It is all about probabilities. Sure, Congress has the power to cut benefits for current and soon-to-be retirees. But I don’t think they have the political will to do so. A much more likely scenario is that there would be some combination of tax increases/benefit cuts (aka means testing) for those under 50. Low probability event, in other words.
On the other hand, over every 8-year rolling period, the S&P 500 has had a negative real return 11% of the time and the median eight year real return is 6.7%
Compare that to guaranteed 8% plus inflation you get by delaying. Spunk your kid all you want, but one of these scenarios has much better odds.