There have always been and always will be misinformed and poorly educated people who believe all kinds of ridiculous things with no factual basis. A healthy democracy doesn't deal with these people by declaring their opinions illegal.
Once you have the state deciding and enforcing what is and isn't true, any hope for a liberal democracy is gone.
There's a line between having an opinion and spreading verifiable lies, and maybe we could do something about the latter category. But yeah, the core of it will probably have to be grassroots, not a government intervention.
Sure there's a line, but it's extremely fuzzy and subjective. Everyone sees it in a different place.
Once you give the state the power to arrest people for spreading "verifiable lies", you now have to determine what "verifiable" means and who does the verifying. It's inevitably going to change a lot depending on the political beliefs of those in power.
It's the halting problem, "who watches the watchers?", etc. There's no way to have the state determining what's true and what's a lie, even in the most seemingly obvious cases, while maintaining freedom of speech and a free society more generally.
No, but it's usually the unsaid implication when stating that certain opinions, ideas, or media outlets are threats to democracy and "we need to do something". Perhaps instead of arresting, it's fining, or silencing, or some other method of enforcement. Regardless, the counter-argument remains the same.
Doesn't mean it's incorrect, though. Doing something is tricky, for sure, but doing nothing is fatal.