Back to one of your earlier points. I think it's obvious who gains from being able to say racist things without fear of arrest: racists do. I don't think anyone else does. But your conceit is to say "Hey, we all think you're wrong. In fact, we think you're so wrong that you can't say what you've been saying anymore." That's what I think is immoral. You can say "We think you're so wrong and we're going to explain exactly why you're wrong" or "We think you're so wrong that we'll reject you socially for your bigoted views" or "I think you're so wrong that I won't hire you". But what right do you have to say "I'm going to throw you in jail because you believe something and then you said it"? Because guess what? You're guilty of the same crime. You believe something (maybe you even have some unpopular beliefs), and you've probably spoken up about them before.
I fail to see how not arresting someone = giving them a platform. I'm all for not publicly funding racist, bigoted people. But why does not criminalizing someone's speech indicate that you endorse what they're saying? Further, if you, as a society, so strongly disagree with hateful speech and don't want anyone to feel like that's a commonly held or accepted view, why don't you just show them that? If there are more of you than there are of them, drown out their voices with your own. Be a model of equality.
Finally, you mention future evolutions of hate speech laws - where exactly do you think they'll land? You mention transphobic people. Do you think hate speech laws should cover that? What else? Where's the line? Would you advocate for speech laws that cover something you believe? What if you're a communist or socialist and everyone thinks that's completely immoral and wrong? Should you be unable to speak up about it?