Until, or unless you can show that there were credible threats emerging from Parler, I don't see any reason to shut it down. I think it unlikely, credible threats would be discussed on an internet notice board, and this is just people - either testing the limits of free speech, or blowing off steam. If there were credible threats, where are the arrests of these potential terrorists? No, this is closing down right wing spaces - in accord with the left wing, politically correct, cancel culture playbook. — counterpunch

You think it unlikely and yet it appears to have happened.

https://beta.documentcloud.org/documents/20454504-eduard-florea-case

We'll probably see more of it over time now that Parler is working with the FBI to provide information on its users. It isn't known if people, who have been arrested through other methods of identification, have been users of Parler and what they posted there. Investigations take time, which answers your "where are the arrests? "; they're under way and not necessarily being identified through Parler data.

The idea that these people are just innocently testing the limits of free speech or blowing off steam is a curious interpretation of calling for assassinations and violence.

This is blowing off steam:

Jesus Fucking Christ I can't FUCKING UNDERSTAND why people can be so fucking dumb and stupid. All politicians should go to hell and fuck of and die.

This isn't:

On January xxx we need to start systematicly assasinating #liberal leaders, liberal activists, #blm leaders and supporters, members of #nba #nfl #mlb # mainstreammedia anchors and correspondents. I already have a news worthy event planned.

The fact you can't or won't tell the difference worries me.