[Lavery]

None of what you're saying is wrong, but remember the context here when using the historical performance of markets to talk about these kinds of hard-and-fast rules. The period for which we have market data also spans: the period with the fastest growth in global population, including population growth in developed markets; the outbreak of extended peace between world powers; and roughly tracks human exploitation of fossil fuels.

Which is not to say that this is for sure coming to an end (hopefully not peace!), but it's impossible to know that there will be similar growth going forward. Population may level off (in advanced economies, this appears to have happened), and while we may well find new energy sources and technologies, it's not likely any will offer the massive productivity gains seen in the advent of fossil fuel and the the internet.

[ThrustVectoring]

If things are bad enough that the 30-year treasury outperforms global stocks at maturity, then the correct hedge is canned food, guns, and ammunition. The most pessimistic long-term outlooks cannot be mitigated by any sort of market mechanism because that level of pessimism implies a breakdown of the market itself.

[Lavery]

But what you're talking about isn't guaranteed. There's a lot of room between "stocks rip higher forever" and "societal collapse".

I'm not saying the right call is 100% t-bonds, but when you say things like "Over something like 30 years, the only question is whether your investment will outperform inflation or massively outperform it", you're making guarantees based on data from a narrow slice of history that is also the most economically favorable period just about ever.

Equities are the residual claim on assets, and their outperformance is predicated on growth. Economic growth comes from growing population and productivity. Neither of those is a given.

[ThrustVectoring]

There's no world in which economic growth is low enough that 30 year bonds win out over equities, yet high enough that the US can afford to pay their debts in full without inflating them away. Economic growth isn't a given, sure, but there basically isn't a plan that works for that pessimistic of market conditions. If there isn't enough economic activity to support retiring a large majority of your generational cohort, then unless you save extraordinarily large amounts of money you simply don't get to retire.

[IkmoIkmo]

This is really underappreciated. I'm a big fan of diversification, and it can be very important to lower your risk-appetite if you're relying on your investment income in a short-medium timeframe (e.g. if you're planning on retiring in 5 years, being all-in equity is just silly, or if you're saving for a downpayment on a home in a few years, stock investing isn't the primary instrument to use), but in the long-term (e.g. a 25 year old thinking about early retirement someday) it just doesn't make sense to take too little risk.

And in part it's precisely because of the worst case scenario happens where worldwide stocks don't outperform treasury bonds, you're likely living in a world where your money isn't legal tender anymore anyway, and any stock/bond investment decision you ever made basically irrelevant.

So if decisions regarding the extreme downside risk are irrelevant, you might as well optimize for the upside.

[ThrustVectoring]

Do note that there is a maximum amount of aggressiveness, beyond which you basically guarantee that you go broke while making trades with positive expected value. If you flip a coin that wins you twice as much as you lose, and you bet your entire bankroll, you'll eventually lose the coin flip and with it your entire savings.

For broad-market equity indexes, this point is at roughly 140% stocks / -40% cash. So it's not close to being an issue with current market expectations for a 100% equity portfolio, but generally speaking there is a level beyond which you cannot further optimize for upside at the expense of downside.