I doubt it will play out as you're imagining. Including second-language speakers, there are about 1.5 to 2 billion speakers of English today, which due to British colonialism, can be found across the globe.
Previous lingua francas such as Latin and French, by comparison, had tens of millions of speakers at most during their heyday, and were less broadly geographically distributed. There are more French speakers in the world today than there were when French was dubbed the lingua franca.
It's difficult to predict how English might evolve, but it's unlikely to undergo a significant global simplification. You already get significantly different dialects in different parts of the world. If anything, if English starts to lose its position as a global language, it will fragment further and pick up new dialect-specific complexities as other local languages mix with it.
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.
Previous lingua francas such as Latin and French, by comparison, had tens of millions of speakers at most during their heyday, and were less broadly geographically distributed. There are more French speakers in the world today than there were when French was dubbed the lingua franca.
It's difficult to predict how English might evolve, but it's unlikely to undergo a significant global simplification. You already get significantly different dialects in different parts of the world. If anything, if English starts to lose its position as a global language, it will fragment further and pick up new dialect-specific complexities as other local languages mix with it.