“Nonracists are so useless...if the world was full of them, there would be no civil rights movement.”
This might be true in a narrow sense. But it's equally true to say that if the world was full of architects, there would be no nurses. Does that make architects useless?
It would be so much more realistic - and also so much less likely to provoke people into resenting your insistence - to say "being nonracist is fine as far as it goes, but we really need more active antiracists".
You also have to consider, I think, that not everybody can "use their platforms" all the time to combat racism. What if they are using their platforms for LGBTQ+ rights, or climate change, or one of dozens of other worthwhile causes?
Are they still useless? If so, I think you need to demonstrate why being antiracist trumps all of these in importance.