The critical thing for the platform is that if I want to find the Twitter account for {celebrity foo} I can do so with a high degree of confidence that it will be real.
I think the loss of trust from consumers is the bigger risk, successful impersonations are relatively high profile and people don't like being tricked.
For example, out of the top 100 twitter accounts (https://socialblade.com/twitter/top/100), almost all are musicians, sports figures, politicians or news outlets.
If the top 5 musicians and the top 5 sports figures got together and started posting content exclusively on a new platform, I wonder if it would be enough to cause a gravitational shift.