In Linux there's the "typing booster" IME that works almost exactly like this. As a side effect it handles diacritics and emoji.
As an aside, I don't like it. Without it, some uncommon words take more dexterity to type fast without assistance, but pinyin input has the same issue. And without it I get to do all the micro-choices on what my text looks like; it has a (not distinct but) deliberate style.
All these 'predict the next word' technologies have a deep but insidious averaging impact on style, and the lack of practice produces dependence (de-skilling). The same thing has already happened to the 汉字 languages, though earlier because they've been using predictive technology from the beginning of computing: regional variants of characters are disappearing, and most people can't write intelligibly by hand anymore.