The only way any part of the "USA" stays Francophone is a very reduced influx of settlers from the English speaking parts of the USA. While French as a first language persisted for a long time in Louisiana, in urban and developed areas it very much became a minority by the ACW. After the Louisiana purchase very few settlers arrived who already spoke French, and those who were not already English speakers in general learned English rather than French. It is worth noting that several decades ago Quebec passed a law that mandated immigrants who were not native English speakers had to send their children to French schools, and other measures to ensure French did not become a second language for newcomers. It was highly controversial btw.