There were many lingua francas of which French was supposedly the first global lingua franca. That changed and it became English (from what I understand). We will probably see another language become the lingua franca, so my question is: should it be English? Are there better candidates out there? Why / why not?
The Celtic languages are closely related to European languages such as Breton, the ancestor languages having been developed and spoken widely in Europe pre-Roman conquest.
I’m only being picky because it adds even more support to your (already very fine) argument. You don’t even need that caveat.