I think Taleb's critique on Modern Portfolio Theory is compelling.
"conservative" vs "aggressive" portfolios are only marginally different from each other because they are both exposed to fat tail events. These portfolios pretend you can calculate the tail risks, where the barbell is a bit more honest. You have years of safety in your 90% cash position, and then you have incredibly convex possibilities in the 10%. The trick is, for an average family you probably need to have a net worth of at least 500K for that 10% risky portion to be meaningful.
That said, your typical FIRE portfolio of just buying the total stock market is probably the best, most efficient version of adhering to MPT, and you'll probably do fine in the end.
However, there are still some very fragile properties of this portfolio. Only in recent human history has one been able to simply buy the whole stock market. It's hard to compare todays returns on "the stock market" to even a few decades ago because those returns were based on a world where only a few people had access to that kind of investing.
Where MMM and Taleb agree, your best potential for return is in starting your own business.
Personally, I feel fine staying out of the market, except it kind of renders IRAs/401ks useless.