So essentially we're losing 400 mana, 15 mana regen per 5 and the STACKABLE passive, for 15 AP (actually 8 if you account for the lost mana on the passive) and a modified manaburn version of blitzcrank passive on an item. I feel as though this item will make champions who stack mana and are already tanky (Ryze, Singed, Blitz) even harder to take down, while spam-y champions who benefited from the glass cannon aspect of AA stacking (Karthus, Veiger, and arguably AP Ez and Nidalee) will be taking a significant hit from this change.
I can understand the possible mana/mana regen nerfs to an item which is considered overpowered, but to my knowledge AA is more of a niche and underused item, forgone for more tanky items like RoA. It seems as though the intent of the redesign to make the item less niche and more generalized. Also what was the design reasoning for making the "Insight" passive unique instead of keeping it a stackable passive like AA?