This move is ignorant to the needs of accessible users who rely on shape and outline as well as colours and inner shapes.
For the sake of everyone, I hope Google will think about and research this more, because while those with highly visual needs will find this more problematic, we all are subconsciously processing the outline shape of an icon, and there is a happier balance between consistency and inconsistency than the one that Google is forcing here.
To make an extreme case as an example — why only provide uniformity in shape, why not colour as well? Why not provide a colour filter that the UI could apply and make all icons green, or blue? These are important visual signals to the end-user, and I am saddened that Google has not considered this.
Where does the drive for uniformity come from? Have you “over-solved” a problem?