You're assuming that fragmentation will occur under the same conditions that affected Latin or French, where there was limited educational infrastructure, no formal institutions to guide the development of language, no global economic pressure for standardization and no instantaneous global communications infrastructure. The feedback loop for standardization is much shorter now, languages are continuing to evolve but that evolution reaches global awareness for synthesis and consensus very quickly. Regional accents and dialects are slowly disappearing and are no longer strongly influenced by factors of regional dominance.

Could there be a reversal of this trend? Maybe, but it seems very unlikely, even with the potential for geopolitical instability from global warming, autonomous warfare, the rise of autocrats and other factors.