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.