Usually nationalism and self-interest. It's why england switched from norman french to english a few hundred years ago. It's also why kazahkstan switched from cryllic to latin alphabet recently and has been pushing for kazakh and english education to the detriment of russian.
> Where is the political benefit in doing so?
To get rid of their former foreign masters' language? As france's power and influence continues to wane, what incentive is there for africans to hold onto a language forced upon them by colonizers? The french language is a symbol of humiliation for a lot of african and asian countries and their elites. Especially the nationalists.
I'm not saying it will happen, but it could. Especially if france declines further and the political and financial incentive to stick to french disappears.