If the actual utility you receive as a function of the total "payout" of your "investments" has diminishing marginal returns, then the character of the portfolio to maximize expected utility depends upon the failure correlations between the investment options.
IE, in the case that the utility function is sufficiently convex to payout and the various investments all fail independently of each other, a strategy of investing in only the highest yield and lowest risk choices is not optimal: a small investment in a middling investment decreases the risk of total failure (and corresponding hit to expected utility) enough to be worth the hit to expected payout.
I haven't run the analysis, but my intuition is that advocacy for a barbell strategy limited to just high-risk stocks and T-bills is an empirical claim about the risks along the following lines:
1) The failure of middling risk stock is well correlated with the failure of high risk stock
2) The failure of less-risky investments than T-bills (paper currency, rice & beans under nitrogen, solar panels, etc.) are well-correlated with the failure of T-bills and have lower annual yields.
3) Utility is a moderately convex function of payout. (If it were very convex, you'd want most or all of your funds in T-bills, not just a bit; if it were linear or concave, "risk" isn't a thing and all funds would be in the stocks.)
I'll trust Taleb on #1, and #3 seems reasonable most of the time, but on #2, it would seem to me that while a good portfolio would be based around the high-risk stocks backed up with a small portion of cash-equivalents, the "insurance" against failure of both of those things is cheap enough that it should be included early on.
(As an aside, I'm pretty sure that Taleb suggests "mostly" high-yield/high-risk holdings, with only enough T-bill stuff to keep you off the streets if the stock fails. That's not what I'd pick out as a strategy that is likely to cause bad outcomes because you didn't take enough risk.)
Nassim Nicholas Taleb recommends that instead of the balanced portfolio of investments recommended by portfolio theory, we follow a "barbell" strategy of putting most of our assets in a maximally safe, stable investment, and making small, sustainable bets with very high potential upside. If taken literally, this can't work because no such safe asset class exists.
Taleb typically uses US treasury bonds as an example of an extremely safe asset. This can only be true metaphorically. They are extremely safe if your measure of value is denominated in US dollars. But of what use to me are US treasury bonds in a hurricane, or if the US dollar collapses? Of what use if people like me become targets for state persecution, stripped of our financial assets or ability to access them? Of what use if I'm trying to deal with a spiritual or health problem and don't know how to find competent advice? If I'm trying to feed myself and don't know how to distinguish profit-seeking propaganda or engineered taste from genuine information about which foods are healthful?
Risks, and opportunities, are many-dimensional. And along very few dimensions is there anything like the sort of crystalline perfection of a treasury bond. A personal network extending to a variety of fields can do a lot to help you avoid getting scammed, and get access to at least an ordinary standard of competence, such as it is. A friend with different interests and skills from yours will do a lot to expand the access you have, and the skills and knowledge at your disposal. Friends from different cultures or communities can serve as an important hedge if you and people like you become a target of extraction or persecution for some reason, or simply if your way of life becomes unsustaintable.
Land you are used to occupying and know well is easier to robustly possess and develop than a rented apartment (but the upside is often higher in high-density areas, which is why young people often prefer them). A spouse makes it easier to pool assets into a venture diversified between building up a single household and seeking external trade or mercenary opportunities. And children who grow up well-adjusted to the world can be helpful if you need to navigate a changing incentive landscape when you've already committed your assets to a bunch of permanent bets (which you need to do, since time is limited). In addition, they may be a decent consolation prize if cryonics doesn't work to ensure your personal survival. Intergenerational communities of interest protect you a lot more day to day, though they themselves can have trouble reallocating resources in changing circumstances.
Risk cannot be avoided, only managed. The risk-free asset is an illusion. Someday your natural life will end, you need to invest taking that into account, and taking into account that this is true of everything else on this earth that you can put your trust in. If you allocate too much of your portfolio to "safe" assets you're leaving yourself unnecessarily exposed to the risk that this asset will become irrelevant, and failing to take the chances that are actually available to improve your position.
To a large extent, young people do well in the long run by pursuing things that are genuinely fun but not universal. This engages our natural sense of opportunity, which doesn't understand formal systems like money very well, but understands very well how to look for high-upside bets that are actually good for us. I wish someone had explained this to me fifteen years ago. If this advice helps you, then think of me when you're successful, and remember that I didn't charge you for it at the time. And remember to pay it forward.
Or, as Ecclesiastes says: