Maybe I'm wrong, but I get the impression that a fair number of those commenting here haven't really spent much time digesting the info on Tyler's "Portfolio Charts" web site, and that perhaps fewer still have read William Bernstein's "Deep Risk" or "The Permanent Portfolio" by Rowland and Lawson. In my opinion Tyler's site is a phenomenal and in many ways unprecedented resource for looking at what it's like to actually live with the returns from any number of portfolios, and is particularly valuable for folks living off of their assets who aren't in a position to weather long drawdowns.
As for the books, the PP book is essential for understanding the logic behind the approach, while that particular very short Bernstein book reflects his lengthy conversations with Mr. Rowland and is a very well-reasoned critiuque of the PP. The Golden Butterfly effectively addresses one of those criticisms, namely that the PP's 4 x 25% approach allocates equal percentages of assets to address threats that are anything but equally likely to occur.
Like a lot of other posters here (not to mention Bogle, Bernstein and of course damn near everyone over on Bogleheads) I personally don't like owning gold, but pretty much all of the optimium risk:reward portfolios on Tyler's site include it.
As an early retiree myself who's lived through the '01-'02 tech bust and the '08-09 crash I've experienced the effects of seeing an extremely diversified, tilted (a la DFA and Merriman), internationally-diversified "conservative" (40% equities) slice-and-dice portfolio nosedive by 25% when backtesting "proved" its worst posssible loss was 8% so I'm well aware of the limitations of the kind of backtesting Tyler's site provides, but one thing it also shows conclusively is what an unnecessarily rough ride anyone who holds only stock and bonds is in for.
An alternative approach that I do find quite compelling is Larry Swedroe's ("Reducing the Risk of Black Swans") wherein a very large slug of bonds (~70% - either pure IT Treasuries or those leavened with some TIPS) are offset with a small dose of only the most volatile, highest-expected-return stocks (international small cap value, emerging markets). Swedroe makes the case pretty compellingly (with plenty of historical data as well as forecasts based on current valuations) in the aforementioned book. Essentially you get a CAGR that's close to a plain vanilla 50:50 allocation but with a much, much smoother ride. One of the PP guys, who goes by the monker of Desert, has implemented a PP-influenced version of the Larry portfolio that (on paper, of course!) looks nearly ideal for a risk-averse retiree who can't stomach large drawdowns:
https://www.portfoliovisualizer.com/backtest-asset-class-allocation?s=y&mode=2&startYear=1975&endYear=2015&initialAmount=10000&annualOperation=0&annualAdjustment=0&inflationAdjusted=true&annualPercentage=0.0&rebalanceType=1&portfolio1=Custom&portfolio2=Custom&portfolio3=Custom&TotalStockMarket1=10&SmallCapBlend1=10&EmergingMarket1=10&FiveYearTBills1=60&Gold1=10Last thought on all of this for now without getting unduly mired in politics is that the PP, its GB variation, the Larry Portoflio and many others are all built around the assumption that U.S. Treasuries will continue to be (in J.M. Lawson's inimitable phrase) "the best horse at the glue factory" when it comes to bonds and will reliably protect during flights to safety/market panics as they did during '08, Brexit, etc. We have, however, seen lots of willing flirting with default as well as downgrading of Treasury bond ratings due to Congressional antics and have now added a Commander in Chief who's on record as supporting such tactics going forward. If such behavior undermines "full faith and credit" and/or the U.S. dollar's reserve currency status it seems to me that all of these "bunker" alternative portfolios are out the window and would need to be replaced by the most broadly diversified, total world market (for both bonds and equities) allocations available.
I appreciate the many thoughtful posters here and look forward to your thoughts.