Asserting that Vriska is childish and must never gonna change is a bit silly considering the Vriska we've always known was literally a child. All childish children grow into adults eventually! And I think at its core the Plot Point arc was much more about letting Vriska become an adult like everyone else than it was about making her a better person. Giving everyone else the chance to mature while Vriska has to keep running around as a 16 year old seems like the far less fair or interesting outcome for her as a character, to me