Manamune has been a core item on Ryze over AA for a long time now. His AP ratios are so low that you'll get much higher damage output by getting MM instead and just auto attack.
Main stats more people build for Ryze penetration > mana > defense. The only thing the change did is make people consider AA over MM because of the addition of the shield, and there's still plenty of testing to be done to see which is actually better.
edit: Another thing to consider is that shields function the same way as bonus health without taking into account armor/MR. That's why shields in general usually have high numbers because they can be shred so easily.