While Nixey's work has come under criticism, calling the whole idea a myth is too dismissive. Once early Christianity became the state religion it acted very aggressively towards anything that could be considered 'pagan'. Take a look at the following article: https://en.wikipedia.org/wiki/Persecution_of_pagans_in_the_l...
At one point the degree of callousness was so high that they even stripped the roof of the Pantheon in Rome for copper [1]. Things like this bring up images of Isis blowing up Palmyra, not the idea of a smooth and enlightened preservation and continuation of the classical Greco-Roman world.
This is apparently a myth. See: https://historyforatheists.com/2020/03/the-great-myths-8-the...