> English is the de facto lingua franca

The fact that the idiom you picked there uses Latin to tell us that English is like French tells you everything you need to know about this status. It's ephemeral in the long term.

I don't know what our grandkids' children will be speaking. It certainly could be English or Mandarin. But languages change, even on this scale. Certainly there's economic value in having a universal language (and a social loss when languages fall into disuse, for that matter), but IMHO that's as far as it goes.