FCUL
FIDELITY US LOW VOLATILITY ETF
stock TSE

It sounds like you're asking if bonds are essentially no longer relevant and no one should invest in them. Like if you were 100% invested in equity but with 40% low vol stocks instead of 40% bonds (or whatever the correct ratio would be)?
Low vol stocks are still medium to medium-high risk. You can select any particular low vol ETF you are interested in and look at the risk profile on Morningstar versus the most popular bond funds, and you should have a clearer idea of what it is exactly you're trading off.
Or maybe you're wondering if you could just be in low vol ETFs 100%? In which case sure, and you can compare the risk profile to the traditional all-in-one of your choice again in the same way.
For example, I'm in ZLB/FCUL/TILV for low vol, and that averages a risk level of 59 on Morningstar. XBAL is 45. So I wouldn't be able to have as much capital preservation, but due to the variance of particular individual ETFs, there is no clear answer as to which will perform best. In the long term, we'd assume fees are going to be the biggest factor between the two.

It sounds like you're asking if bonds are essentially no longer relevant and no one should invest in them. Like if you were 100% invested in equity but with 40% low vol stocks instead of 40% bonds (or whatever the correct ratio would be)?
Low vol stocks are still medium to medium-high risk. You can select any particular low vol ETF you are interested in and look at the risk profile on Morningstar versus the most popular bond funds, and you should have a clearer idea of what it is exactly you're trading off.
Or maybe you're wondering if you could just be in low vol ETFs 100%? In which case sure, and you can compare the risk profile to the traditional all-in-one of your choice again in the same way.
For example, I'm in ZLB/FCUL/TILV for low vol, and that averages a risk level of 59 on Morningstar. XBAL is 45. So I wouldn't be able to have as much capital preservation, but due to the variance of particular individual ETFs, there is no clear answer as to which will perform best. In the long term, we'd assume fees are going to be the biggest factor between the two.