The genie was out of the bottle once you saw cases worldwide from people who did not go to Wuhan. Genetic analysis of SARS-Cov-2 in early cases shows a lot in common with Europe. Most likely was Wuhan - Northern Italy - US, in addition to Wuhan - US. Paris was hit hard early, too. Stopping just flights from China to US would have slowed it a bit, but NOTHING was going stop this.