Rebalancing into bonds and mmmfs is a form of insurance against catastrophic losses equities. But if you have a sufficiently large account then catastrophic losses that affect your life are extremely rare, if they do occur they will likely affect your bond portfolio as well, and the expected loss vs 100% equities over 15-20 years is significant, something like 10x the value of the insurance you are buying.
If you want insurance for a large account then long-dated put options 20% of the money are much cheaper.
US large cap has certainly recently outperformed the other parts of the target date fund (international stocks, bonds). But there is certainly no guarantee that it will happen "every time". In the last 10 years, US equity has been the best overall performing asset class for the past decade but 7 out of those 10 years at least one other category outperformed it: https://www.blackrock.com/corporate/insights/blackrock-inves...
> Like pension funds I bet Vanguard is one of these so-called LPs who give money to VCs like Y Combinator to help ivy league kids follow their dreams.
You can look up the holdings of VFIFX or any other Vanguard fund. There is no private equity or private credit.
I would suggest a step beyond though, because rebalancing your portfolio is fun year 1-5, but not so fun year 5-20: have a look at e.g. Vanguard retirement target funds.
Essentially, it's an ETF with a rebalancing rule included for a specific target date. For instance if you buy the target 2050 (your hypothetical retirement age), the ETF rebalances itself between bonds/monetary fund/stocks until it reaches that date, u til it's pretty much all cash in 2050.
Lowest hassle diversified retirement scheme I found.