There’s a few rules if you want your system, whether it’s a club, corporation, religion, country or civilization to be stable.
The most important however is that you must not give power to those who want to change your system.
Simple enough.
Now, New Deal/Post-war Liberalism did a few things right. One of them was high marginal tax rates and another was high estate taxes (though not high enough.)
But the new Deal made a devil’s deal: it allowed large corporations to exist. This wasn’t, actually, FDR’s first choice, but he was having trouble fixing the Great Depression, and this is where the solution set wound up.