ER wasn't good for all ADCs. There are "power ADCs like MF and Sivir who have better scaling, lower CDs, whose damage is more reliant on their skills than straight auto-attacks. To them ER was pretty vital to maintain constantly using your abilities and keeping you with enough mana to be able to contribute to any fight that happens to start. There is more to that problem though, the ER lost a lot of its power to such ADCs.
The change to IE is 100% first-make mandatory now thanks to a couple of other changes. The biggest one is to Lord Domick's Regard. They completely took out the bonus damage based on health effect, basically crippling our ability to effectively deal with the tank/high health pool meta. Combine that with IEs change and it pretty seems like if you want to effectively deal with 2 - 3 3K health team comps you need the IE, as much of the armor shred items (ghostblade, duskblade, Black Cleaver) and make sure your crit is +50%. Only way I can think of, on paper, to be able to pump out enough damage on such a comp and be able to work. This is classically Bronze BTW, I don't care enough to focus on trying to climb but I know when you get games the enemy team has 2 - 3 fighters who get a couple of health items literally presents you with too much health to deal with. If a fight works well you may be able to take one down, but by then the rest will have crippled killed the rest of the team and then you SOL.