uhm...
mages aren't bad.
I don't think you're really looking at this from the right perspective? There isn't any possible way for ER to exist for mages.
What would the trigger be? How would the mage player get mana back without it just being passive, free sustain? Because that's Riot's issue. Essence Reaver is fine because you can still easily run out of mana if you don't get to AA. I mean, for example, on a character like Varus it's easy to still have mana problems with ER because it can be hard to get into AA range safely. ER's sustain paradigm requires it though. What is the equivalent for AP? Because that's what Riot is introducing. These new mechanics (Awe reducing mana costs by 25%, the new catalyst passive, new chalice) are targetted at providing mana sustain that involves interacting with your opponent rather than just free mana regen.
The main difference is that Athene's purchases were intended to stall the game out - "I need to waveclear and keep waveclearing without having to back for as long as possible, so Athene's." ER encourages aggressive gameplay - "I have access to crits turning into mana, but only if I'm actively AAing." Riot is attempting to give mages options that give them similar kinds of sustain, but because they ONLY rely on abilities, giving them just another mana regen item that would replace any other mana regen item is silly.
Also, it's not "MASSIVE" stats. the only other final tier item focused at ADCs with the same AD as it (65) is IE, and IE has the crit damage passive to help it. All the other ADC/AD items are 75+ AD. 20% crit is in line with IE's crit and less than the upgraded zeal items... as far as pure combat stats go, ER loses to almost every other ADC item, with the CDR requiring at least 1 investment of 2.8~ gold additionally to hit the bonus CDR cap. In fact, 10 times out of 10 making Essence Reaver is a (fairly significant) loss to DPS over making IE.
Basically, you're mad that Mages don't have ER and claim it makes ADCs have unfair mana options, but their options for mana management are 2 items (Manamune and Essence Reaver) while mages have many more options (Morello, Archangel's, RoA, Athene's) to build both their ability to mana mana AND their pool simultaneously (with stuff like catalyst passive giving both mana pool and the mana/HP sustain or Tear giving both 1k max mana pool fully stacked and 25% reduced costs, with new changes coming in). Yeah ADCs have access to Tear as well, but their ability to increase their max mana pool is greatly reduced, meaning that they can't engage in an endless poke paradigm with ER, which is what Riot is trying to prevent with mages.
tl;dr: ER or an item like it will never exist because the mages that would be able to abuse that kind of mana sustain for an endless poke paradigm would be stupidly broken or require nerfs (Ziggs) as we've already seen with the release of any major waveclear midlaner who can opt into athene's for eternal waveclear.