AGGY
WisdomTree Yield Enhanced U.S. Aggregate Bond Fund
stock NYSE ETF

The main problem with a passive bond fund like BND is that the benchmark index AGG (Barclay Aggregate bond index) has changed significantly over time due to growing deficits of the US government. AGG 10 to 20 years is vastly different from today. This is a problem which will continue. The index also lacks a significant part of the investable fixed income markets that have grown significantly over time as they developed.
Lastly, there is the question of whether AGG is a good bond index to use for retirement investments and to balance a stock heavy portfolio. AGG is a cap weighted (but limited) index which frankly is a bad idea that led to the index composition changes over time. There is no inherent reason why AGG should be preferred over other indices that are more complete.
You don't really need heavy active management for core bond holding in my view, as they will be mostly or entirely IG-rated debt, but I think a better index is crucial. There isn't a lot of risk or alpha to be had from IG debt to begin with. There are numerous funds that try to do exactly this, like AGGY and many core plus bond funds are that are not all that active in practice and fairly low in fees. Light active management or a better index with a low fee is the way to go in my view.
For non-core holdings like high yield, distressed, bank loans, CLOs/BDCs, asset backed and so on, a slice of which could enhance your fixed income holdings, yes, active management is more important and even required in many cases. Othet than for HY, there is no real investable passive option due to the nature of those assets. If there is an index, it often won't be good or complete due to the lack of transparency in those markets and not investable or easily replicated due to illiquidity.

The main problem with a passive bond fund like BND is that the benchmark index AGG (Barclay Aggregate bond index) has changed significantly over time due to growing deficits of the US government. AGG 10 to 20 years is vastly different from today. This is a problem which will continue. The index also lacks a significant part of the investable fixed income markets that have grown significantly over time as they developed.
Lastly, there is the question of whether AGG is a good bond index to use for retirement investments and to balance a stock heavy portfolio. AGG is a cap weighted (but limited) index which frankly is a bad idea that led to the index composition changes over time. There is no inherent reason why AGG should be preferred over other indices that are more complete.
You don't really need heavy active management for core bond holding in my view, as they will be mostly or entirely IG-rated debt, but I think a better index is crucial. There isn't a lot of risk or alpha to be had from IG debt to begin with. There are numerous funds that try to do exactly this, like AGGY and many core plus bond funds are that are not all that active in practice and fairly low in fees. Light active management or a better index with a low fee is the way to go in my view.
For non-core holdings like high yield, distressed, bank loans, CLOs/BDCs, asset backed and so on, a slice of which could enhance your fixed income holdings, yes, active management is more important and even required in many cases. Othet than for HY, there is no real investable passive option due to the nature of those assets. If there is an index, it often won't be good or complete due to the lack of transparency in those markets and not investable or easily replicated due to illiquidity.