If you start at a company today the default setting would be for the 401k money to be invested in something like LifePath N or similar ETFs that are designed for retirement funds. Most people, lacking the knowledge to do otherwise will stick with that.
So you are working on the assumption that LifePath or similar ETFs are going to be correctly managed for their cohort (shifting into safer investments as the retirement date approaches).
Somehow the "safety" of bonds were not adjusted as yields reached and sometimes went below zero. A multi-year zero interest bond is not a very safe investment. I did have that default setting on a retirement account and got out of those LifePath type ETFs over a decade ago.
You appear to be confusing safety with investment returns. As an asset class, bonds with high credit ratings have a lower risk of losing capital than stocks. Those bonds will lose some value as interest rates rise but they very rarely go to zero. For investors approaching retirement age it's more important to preserve capital than to worry about interest rate risks.
Nevertheless, I find target retirement date funds pull out of stocks and move to bonds way too early. I’d probably only start moving out of stocks 2-3 years before retirement, based on how long recessions/crashes of the past lasted.
Historically, it looks like the risk of being in stocks as you approach retirement is pretty low. Let’s say you planned on retiring in 2009 at age 65. 2009 hits, stocks fall 50%… if you just continued working for 2-3 more years, your retirement account would be back at fresh all time highs AND you’d be collecting a larger SS paycheck each month when you retired at 67-68. Not exactly tragic.
Total market index funds don't go to zero. You would have a bond fund and stock fund and rebalance to more stocks as you get nearer retirement. Sure, but maybe not when there have been very low interest rates for a long time. Holding cash in that case seems better than bonds to me. No upside in bonds, just downside.
Bond funds generate higher risk-adjusted returns than cash over most historical periods. You have to factor in income from coupon payments. You can cherry pick a few brief periods when this wasn't true but no one has the ability to reliably predict when that will happen in the future so it's a foolish approach to retirement planning. Most target date mutual funds do include some cash and equivalents in the portfolio as the target date approaches.
Major passive target date funds are all correctly managed. Periodically rebalancing between the various component index funds to match a set ratio is not exactly rocket science. You can just read the prospectus and latest audit.
So you are working on the assumption that LifePath or similar ETFs are going to be correctly managed for their cohort (shifting into safer investments as the retirement date approaches).