They'll either fail to make it big and remain a niche thing for those trolls who enjoy the edginess, like 4chan and its newer alternatives, or...
They'll grow big and end up having to deal with "trust and safety" issues when the majority of users on the mainstream end of the spectrum demand protection from those users who would otherwise be on 4chan/etc. This is pretty much what Twitter and Facebook are going through now, but shifted more towards the right, in the case of Parler/Gab.
We can already see hints of this in the fact that their terms of service prohibit inciting terrorism or acts of violence – issues that 4chan/etc deal with by allowing it, reducing their reach, and Twitter (attempt) to deal with by banning it, increasing their reach as they provide a "safe space".
Parler has the same issues as Twitter, it's just shifted to the right of the political spectrum. They can either deal with those issues and move closer to Twitter, or not and be irrelevant in the long term.