I disagree the philosophy of the founding fathers has failed. It is instead being rejected, which is different. One of the key sentences in the declaration of independence is:

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.

First, we must believe in a higher power than humans (Creator), and the precise detail don't matter. However when humans assume this position, that is when immorality takes over and people think they can rule of others.

Then you have "the pursuit of Happiness". This has been largely rejected by people who reject the American foundations and the intentions of the founding fathers. They think that some people ought to sacrifice their pursuit of happiness because their ancestors committed some crimes. Post-modernism is meant to destroy everything it touches, and the Constitution, bill or rights and declaration of indepence is no exception. This doesn't mean that they don't work once people are educated on what they mean, since it is not supposed to be a metaphysical philosophy but rather a political one. The only prerequisite is believing in a higher power, even if it's the Universe.

I agree that the success of the post-modernist philosophies lies in the different kind of epistemology it introduces. In other words, it changes the definition of "truth". This just means it can create whatever bullshit it wants out of thin air, and of course it does so effectively gaslighting the masses with lies. The majority of people obviously don't read too much into philosophical arguments and just go with the opinion of the masses, so when post-modernist theories like critical race theory enters academia, it succeeds in convincing the masses with made up lies, which are justified by more lies at ever turn. From there it spread to the general population. I think the way out is an anti-postmodernist philosophy that counters the bullshit, and grounds everyone in the usual intuitive epistemology that doesn't require a Phd in philosophy.