Parler most certainly does have a right to exist. And companies have a right to not associate with a platform that incites violence and foments activity against the very constitution it appeals to.
You can be sure that any companies that host user-published content today are looking closely at the response to Parler, and are probably accelerating plans to get off of AWS. What happens when the Eye of Sauron turns to you and declares that you don't do enough to moderate?
Won't even be a blip on AWS's radar. Aside from rightwingers self hosting, nothing will really change in an impactful way.
Eye of sauron? Dramatic, much? It's not hard: Don't host incitement of violence, i.e. Incitement to Imminent Lawless Action. It's not hard. But for these radical right wingers, it apparently is. Sad.