Technically that did semi happen....or A2 became more like 2B over time anyways and carried out her will regardless.
Comparing A2 in the beginning to the middle and the end is like a completely different character.
And she lovingly stokes 9S's cheek as if she had known him in the same way 2B had, i think its obvious that they assimilated, especially considering that to these androids, memories are their entire being, their souls. without them they are just another model number
Having A2 absorb and assimilate 2B's memories through her sword, it makes sense and is sad in a way that 9S was attempting to destroy her so earnestly