but we have to remember the "i can't breathe" warmups and activity surrounding Trayvon Martin didn't actually results in any policy changes
This is a tough one, even with these protests there's not really a specific policy to change. It's not like there's a "black lives don't matter" policy that needs to be repealed. It's more of a collective idea that needs to be continually pushed forward, something that the I can't breathe t-shirts probably helped with.
Daniel Pantaleo (Eric Garner choke hold) fired.
Amber Guyger (shot black man in own apartment), convicted of murder.
Aaron Dean (shot black women through window while she was in her home playing video games), charged with murder.
Derek Chauvin (George Floyd), charged with murder.
I would argue all the small protests over the years, the I can't breath shirts, the Colin Kaepernick protest, etc. all helped push us here and contributed to those officers being fired/charged with murder. All those small protests beforehand helped raise public awareness.
If the NBA plans to sit out, I think they'd need specific, accomplishable goals. "We won't play until qualified immunity is repealed" something like that. Still think they could do more good by playing, and doubt protests will still be going on in August (so NBA will no longer be distracting people from the issues), but that's just my own guess.
In 1968, Kareem boycotted the Olympics. John Carlos and Tommie Smith played. I think Carlos and Smith's actions did more to further the cause then Kareem's boycott. Unfortunately it was a long slow process though, and at the time it probably seemed like they accomplished little.