Since we're including very broad analyses in the discussion, let me comment on the house of cards thesis, and the up-and-coming-capitalists response.
In my view the essence of the house of cards theory is that debt (or some other quirk of our particular system) is so critical to our society that once it cracks, the entire system fractures and then collapses. Presumably the capital markets would then fail to deliver returns of 4% on a Trinity-style portfolio of stocks and bonds. Put differently, adding debt has an accelerating effect temporarily, but later is fated to have a negative effect that will be stronger than the acceleration, so strong that it invalidates the "4% rule."
I suspect this is incorrect on two levels. Returns may fall below 4%, but if they do, it will probably not be because debt was a fatal flaw, which if avoided would have allowed 4% returns to continue.
The first level of incorrect is that debt is part of the surface expression of our society's wealth, and part of the mechanism for arranging our work, but it is not a fundamental part. Developing new technology and maintaining old technology, growing food and maintaining tools, productive activities - these are wealth creation. Debt, credit, stock, accounting - these are measurement tools, communication tools. They are key parts of how we choose to arrange our labor, but the labor itself creates the wealth. If all the debt in the world disappeared and everyone woke up the next day and grew food and drove trucks and handed food out to people who went to grocery stores, everyone would still be able to eat. Sure, people are not going to change overnight the mindset of how we do things, so yes a debt collapse will temporarily cause a slowdown, but the fundamental economy underneath remains.
The second level of incorrect would be that debt (or any other quirk similarly discussed) is so sneaky and two-edged a sword that when it goes bad instead of good, it will be so powerful as to destroy us, or at least the 4% rule. I suspect that our global economy will always have elements of instability, but then also develop responses that provide acceptable stability. What we survive, we will learn to handle. Since as discussed above we are going to survive, when debt (or whatever) periodically triggers a bad period, we will respond in ways that address that particular error. If we fail, we'll learn the lesson twice and do better the second time. My guess is that the process won't be so bad as to invalidate the 4% rule...though it must be noted that the 4% rule only projects a 95% chance of being adequate for 30 years, not 100%. Occasional dips under 4% are expected!
Re the up and coming capitalists - I agree they are a strong force sustaining future returns, probably strong enough to overcome debt headwinds. Not strong enough to make the 4% rule stronger or weaker than before is my guess.