I agree that the "black on black crime" rebuttal doesn't make sense if someone is saying it in the sense that "black people need to clean up their own backyard before they complain about anything else". That's dumb. Injustice is injustice and while resources are limited, we can shed light on all kinds of problems in this world.

That being said, I think most people are simply pointing out one possible irrationality of black lives matter, which is that they are focusing on cop-on-black crime and ignoring black-on-black crime. I'm not saying this is the reality, just saying this is their argument which is different than the "clean up your own backyard" argument.

Now I'll respond to your first 2 points.

#1 I agree, ending the black-on-black crime problem is really like trying to end all crime. It's a very difficult proposition. But I think you can easily apply that same logic to the police brutality/racism problem. It is also an insanely complex problem, but it doesn't mean we shouldn't talk about the problem or try to solve it.

#2 Police brutality and racism isn't that bad. It was way worse in past decades...

See what I did there. If it's a problem, it's a problem. You can rationalize it away by saying things have gotten relatively better. Just about everything in terms of violence and standard of living is relatively better than the past for most people on the planet. There are still serious problems that need to be addressed.