Woa. Relax buddy. I was Plat II in my prime. That's relatively high elo. I just don't play as much anymore and quit for a while so my mmr and elo dropped. I did build athenes before it was changed into a more support based item. The healing effect is great indeed like you said, but every game I have played, my damage output took greater priority over the negligible healing.
Indeed it has been her core item for a long time, but that doesn't mean it will remain so if they change it like that. I prefer damage. It's just a personal preference. Her greatest use to her team has always been and will always be reliably landing a good ult. With athenes, you do less damage so landing a good ult doesn't help as much as if you build more damage.
As for mana regen, Morellos doesn't need to give her good mana regen, it gives her 400 mana. That's a lot of extra mana to use. I have since learned to mana manage with her and any other mid laner I play. Even without blue or mana regen items, I still always have enough for a team fight or a fight.
As for having the mechanics to use it, yes I do have the mechanics for it. I land my ults reliably, my ball goes where it needs to go and I generally end up doing really well both in and out of laning phase. Being low elo has nothing to do with having or not having mechanics. Other diamond level players have suggested that they are quite surprised at the skill of certain lower elo players while they are smurfing.
You're right. Not every mid laner should do damage, but the team comps I get into are generally low damage team comps. Usually the top, jungle and support are tanky or low damage output leaving only the adc. If the adc is unreliable, I need to be able to deal damage so I can at least chunk health for our adc to finish off.
Finally, I want to finish by saying that I have tried the new Athenes a few times, but didn't like the result. My team didn't benefit from it as much as I would have liked.