Now that Durov is in custody, one can only ask what exactly French authorities hope to achieve with that move. It's unlikely that they really believe that the owner of Telegram is responsible for the existence of pedophiles, drug dealers, terrorists, and other criminals, so whatever they say it's clear that his arrest is an attack on free speech and its advocates.
It should be enough of a red flag that Russia and the far-right have joined forces to echo the argument that the arrest of Durov is an attack on free speech. Actually, the investigation is focused on standard crimes committed on his private platform and he has been detained for questioning. Durov is presumed innocent and will be able to defend himself if indicted.