I've been thinking about the crit changes a lot today, and my conclusion is that they are taking power out of marksmen late game to let them have tools like this.
If you want 100% crit on full build, it takes 4 items. You only get one more, and you have to choose whether that's going to be lifesteal, armor pen, or a defensive item. Therefore, I think marksmen are not going to have 100% crit in many games, even when they go late. In particularly rough games, they might even have to settle for 50% crit until very late.
In exchange, since their offense is overall down, they're gonna need some help staying alive a bit longer. At least, that's the theory.