True, but incredibly overstated and overused. It's mostly used to excuse intellectual laziness on all sides.

The notion of objective truth has a long and effective history at obviating this problem: you share your reasons, their support, relevant facts, discuss why they're sufficient to make the case (or not), and eventually get to either a disagreement on facts that cannot be easily supported by evidence, or a subjective opinion that neither person can justify, or one person convinces the other, or one or both get sick of it and/or cannot or will not use valid arguments any longer and the matter stays unresolved.

We have the cognitive tools to dramatically reduce the biases; we shouldn't excuse ourselves or each other from using them.

For instance, if your bias is that you'd like more money and fewer cars, and your narrative is that you have a car for sale for $10,000; and my bias is that I like both cars and money, and my narrative is that you're giving your car away and I take it without paying, we don't have "conflicting narratives"--certainly not with equal validity. I'm confused or lying, and I stole your car.

It's true that persuading people with reasons is difficult, and it's more so on short-format social media with algorithms that enrich for shock and outrage (two things long recognized as impediments to reason). But under the right circumstances, rational arguments do work: https://lifehacker.com/can-rational-arguments-actually-change-peoples-minds-1590008558

Regarding context, recall that your section title was "‘But Black On Black Crime’ is not a reasonable or good faith response by anyone" and you didn't specify precisly what it was a response to, just something to do with Black Lives Matter. And having read your previous reply over again, I think I was inconsistent in what context I was envisioning in each case.

I don't find your arguments convincing regarding stealing of attention, for two reasons. I think you overstate the case when you say

This is classic whataboutery - the tactic of pointing to something different to dismiss one cause. 'Oh so you're fighting for X? What about Y?'. In every instance, whataboutery is fundamentally bad faith.

The argument you have to support this is

Policy makers, faced with resource constraint prioritizing more pressing problems is one thing, the average citizen responding 'oh you care about black lives? Then what about those lost in crime' is a remark made snidely, not out of genuine concern. If only because obviously, when it relates to crime, the expected response is from the State. Not the citizens.

But in a democracy this is patently wrong. Policy makers are supposed to be, and indeed are structurally required to be (to an extent), responsive to the concerns of the citizens. It's why people take polls of "what are your top concerns for 2022" and so on.

This isn't like running a pet shelter for kittens, where you can reasonably expect that the individual discretion we give to people to organize their own efforts covers matters. You don't say, "But what about injured birds!"...well...you might, actually, but if the person just really likes kittens, "Because I love and know how to help kittens, and I have neither the skill nor interest to help birds," is a totally fair response. It's such a fair response, and such a likely one, that it's unseemly to even ask the question.

But BLM isn't like a (single) pet shelter. It has had a huge national pull. It can absorb a great deal of attention, nationally, so much that it's at least plausible that it has taken some attention off of other things (especially if you consider the full decentralized movement). And it is mostly framed as a moral issue, as it should be: it is evil for a state to hold the lives of some of its citizens in callous disregard, even killing some of them directly, and not seeking justice when others are killed. But there may be moral consequences of certain changes to how the state conducts itself, and those are worth discussing. The Black Lives Matter movement has power. With power comes responsibility, including being aware of any incidental negative consequences of the application of your power. Someone might be wrong about whether there are negative consequences, but is it also wrong to want to talk about it?

I think one could introduce the topic in a less insensitive way that just raising "black on black crime" as a counter to "Black Lives Matter". So as a rule of thumb, I'm with your initial and continuing characterization.

But I still think there are exceptions (not just in the abstract--I've seen things that look to me like they're exceptions of this sort), and this should serve as a caution against judging too harshly too quickly.