Most 401K fund investing from people age 30 and under is retirement date funds, that for a retirement 2050 looks like the following: about 3% in foreign bonds of 9 percent in total
By the time you get to 5 years from retirement the holdings are:
Vanguard Total Stock Market Index Fund Investor Shares 37.00%
Vanguard Total Bond Market II Index Fund** 27.40%
Vanguard Total International Stock Index Fund Investor Shares 24.20%
Vanguard Total International Bond Index Fund Investor Shares 11.40%
100.00%.
https://twocents.lifehacker.com/when-to-opt-out-of-the-target-date-funds-in-your-401-k-1830435912
"In fact, “68 percent of millennials are 100 percent invested in a target date"
Almost all new money being invested by young people is going into target date funds and so the allocation to foreign bonds and equity is only going to increase over time. This will allow negative bond yields to become the defacto choice as money to invest in them will be growing, leading to an increase in their percentage as their market cap appreciates faster. Indeed when a central bank purchases their long dated bonds at very low interest rate, this causes funds to buy MORE of their bonds not less because as a percent of the index they are climbing, leading to even lower rates which continues the process, and Central Bank buying leads to a shortage which leads to higher prices which leads to even more purchases, it is utterly insane yet true....
As long the young investors do not pull out their money, this means stocks and bonds will have the ability to go to unimaginable heights, and the circumstances that led to no interest bonds going up 50% in a month will continue and spread to stock because any sales by a value investor because of expected returns is overwhelmed by the new young investors who are 90-96% committed to passive investing and have a holding period of forever, making active managers looking at expected returns look foolish and losing their jobs as their funds dwindle. In such a universe prices will go up in a self-confiming bias, until there is a market moment that causes investors to wish to lower equity holdings.
https://www.cnbc.com/2018/06/12/mil...and-flocking-to-etfs-schwab-survey-shows.html
Presently this is not as big of an issue because older retirees are less invested in passive investing and active investing holds a larger part of the assets right now but as the passive pool becomes larger than the active pool, the lack of value analysis by the holders of equity will reduce the amount of stock available for sale at a price at the margin and reduce the amount of buyers at a lower price. Years with 50-60 percent advances in the coming decade I believe are a very real possibility, as is the possibility for 50-60 percent down in an incredibly short period.