Even in the US freedom of speech is not freedom from consequences. quite the opposite. I'm sure in Nazi times in europe, being critical of the Nazis wasn't allowed speech. Both then and now in the US, nazis prosper because we didn't use free speech to fend them off enough, we were complacent. You can see trump trying to retaliate by firing people, banning journalists, cancelling contracts, etc.. for anyone critical of his administration. speech is how fascists gain power, but opposing speech across europe (look at UK and France) is also how they just recently fended of right-wing fascists.
In a democracy, the government gets its power from the people. if it can silence the people in certain contexts, then in those contexts the people lost their power. When politicians with bad intent take power, they'll use this crack in the system to tear it apart.
That's where I think you're wrong, they won't use a restriction on speech to arrest someone, they will do anything because there won't be consequences.
The current US administration is trying to officially get rid of habeas corpus, even though the government has already arrested people without due process.
Even when freedom of speech is fully enshrined into law, anyone with bad intent won't care.
In a democracy, the government gets its power from the people. if it can silence the people in certain contexts, then in those contexts the people lost their power. When politicians with bad intent take power, they'll use this crack in the system to tear it apart.