Personally, I think a football player sitting during the anthem is ineffectual. It’s less effective even than temporarily adding an overlay to your Facebook avatar after a crisis.
If the people who sit want to help the cause for which they protest, they should do so in a way that matters. You can’t show solidarity to the BLM community by sitting unless the camera notices you doing it and the media jumps up and down accordingly. So I think the people who sit should be ignored, because it deflates their method of protest entirely.
It’s similar to how Trump wouldn’t have won the primary had he not been given all the media attention. Just ignore them, and their opinion becomes moot.