Deplatforming, as we're calling it now, does work. The problem is you misunderstand the strategy. Of course, when they're kicked off one host, they'll pop-up somewhere else. However, each time they're taken offline their community fractures. Some leave, some start competing services, by preventing the community from congregating in a single place they are prevented from obtaining critical mass again.
A good comparison is looking at what happened in the Pirating world. ThePirateBay used to be the only to go to place for media. Then they were continually shutdown, moved around, shutdown, etc. During that time, the community largely fractured, now while the TPB is back, it is a shadow of its former self, and finding specific or rare media requires searching through a dozen or so various torrent sites.
The parler/right-wing community will face the same problem. Eventually, Parler will learn how to stay online, but by that time their community will have shifted among a dozen or so forums and apps.