This is often repeated, but I have not seen evidence of it. While it's true that stocks loose value faster, my experience in , when was it, 1987, when we joked our 401k turned into a 201k, my bonds dropped like a rock, just not as far as stocks. When there's a flight to cash, there's no asset class that's not getting impacted. Nothing zigs when stocks zag...that would be too easy. I do buy into the risk adjusted return argument, so hold a guaranteed income fund. That allows me to buy equities low, when everyone else is running for the hills.
The way I've understood it from reading books and articles from many financial folks - Bogle, Clements, etc, is that bonds & equities move independently of each other. Not opposite. Big difference. Some years both tank. Some years, both thrive. Other years, go opposite ways.
Bond funds must also be matched to the right time horizon for best results: I think many people were told intermediate bond funds are "safe" investments. They are in the long term, but definitely not for near-term spending needs - i.e., next 3-5 years. Even short term bond funds aren't ideal for 1-4 year near-term spending vs CDs, MMs, Tbills.
Storing funds in mm/cds sure didn't feel too good to me when rates were 1% and I was chasing .10 changes in yield - which wasn't that long ago. Cash instruments look appealing now - but that can sure change fast.
And as brutal as bond declines were, the drops still tend to be lower than those brutal stock bear markets: most bonds didn't tank 30-50% Stocks can and eventually will have those really bad days.
Many who hold 90-100% stock allocations have a pension, inheritance or RE income - but may not disclose that safety net when advising others to go heavier in equity.
I'm not loving bonds either right now, but in a diversified portfolio that I must live on, something is always down, while something else is up. I live with that volatility and look long term.