Where's the part where they act on these detestable ideas & we're powerless to stop these acts & hold people accountable for their actions? Behaviors are acts (distinct from speech) and I see only claims to defend speech.

Unless you exterminate everyone you disagree with, people with ideas you disapprove of will always exist. Better to know who they are by letting them tell us. Civil liberties & a right to exist apply as much to them as to you.

As you wrote, people are malleable. They don't need the input of others to develop incorrect ideas & common biases on their own especially from an early age. As that article on early childhood development of racial prejudices points out, avoiding talking about discriminatory biases or delaying the topic is not the answer. Early intervention with active, explicit conversation is important to correct biases & misconceptions acquired from implicit social factors, which suppression of speech will not prevent. With appropriate work, people can & often need to be corrected.

Agreement through suppressing opposing ideas is unreliable & inadequate. It doesn't correct self-learned biases. It assumes people will only hold unopposed ideas, which indicates they never reliably held them. If an idea has any merit, people should hold them despite flawed challenges, because we did the work of educating them properly & they know better. Choosing to compromise freedoms instead is flat out lazy & an insult to everyone's dignity.

Finally, it's pretty asinine to assume we need to sacrifice civil liberties to gain civil liberties. In the United States, the free speech & civil liberties movements gained together. That happened despite worse racism then with Jim Crow laws & white supremacists speaking freely. If we were able to gain civil liberties then under harsher conditions, then we shouldn't have to sacrifice them now.