Spot on with your argument. Awesome job.
No powerful group (white people included) has ever made concessions on its social power because it was persuaded to do so with arguments of any kind, let alone moderate ones. They can only be forced through protest and agitation. If “reasonable” (read: overwhelmingly white) voters want the left to become more moderate, it’s because they want the world to be more comfortable for progressive, cisgendered white people without actually conceding any of their power. If one of these “reasonable” people tells you to become more moderate in order to win elections, you can be pretty sure that the impact of actually winning those elections would only be felt by the most privileged liberals, and never the vulnerable ones. For them, things will just continue to be the same as they’ve always been.