Honestly, I'm not sure why you've decided that this sort of thing has no net positive effect when all the evidence is to contrary. One shining example is Martin Luther King. His rhetoric was considered inflammatory and divisive in its day and he was widely hated for it — and later murdered. Nonetheless, nearly every thing he advocated for was eventually codified in the Voting Rights Act and the Civil Rights Act. Looking forward to the protests in the wake of George Floyd's murder, a huge number of positive change came out of those, including some things that had been being advocated for for decades with no effect - such as The Washington Redskins finally changing their name and NASCAR doing away with the Confederate flag as well as dozens if not hundreds of other net-positive changes. We still need more, but those protests did push progress forward.
This process is not pristine. It is not without issues, occasional oversteps, and other problems that arise when human beings are involved, and it always comes with push-back, but it is also the only way that social progress has ever come about. The fact that there are still a huge number of people who will resist that tooth and nail, to the point of being willing to overthrow the government is not the fault of people who were not "nice" enough in their advocacy. And them being nicer is not going to bring about anything other than as Dr. King pointed out, “tranquility and the status quo” at the expense of justice and humanity.
The polarizing of society is caused by two things
1) Most people are not that far off in their middle-of-the road mainstream beliefs and so it becomes necessary to inflate differences in order to garner votes and to differentiate yourself.
2) A comparatively small, but highly radicalized segment of society is deeply disturbed at their innate societal advantages and privileges being challenged and they are willing to do things like over-throw the government to try to stem the tide of that.
Vocal activism and taking to task people who used to get away with routinely marginalizing and mocking groups with less social power is not the direct cause of either of those dynamics. Not unless you want to halt social progress so that the white supremacists will feel better and less threatened, which I don't think is a viable option. As the man said, unless you can come up with a different proactive plan for bringing about social progress in another way that actually works, you're just a part of the problem. "Polite dissent" about someone else's fundamental human rights isn't the measured, sensible thing that you seem to imagine it as. Honestly, my antarctic friend, it may all "feel" this way to you, but the evidence simply doesn't support your feelings.
