Unfortunately, by the end of the Remake, it wasn't exactly clear what Sephiroth's big great plan was. I know people have a lot of different ideas.
The novel explains that her powers are greatest when she's close to soil that is rich in mako/Lifestream, and that her childish mind also made it easier for her to access them:
Aerith: “I don’t think it was just my powers, but I think the land had an influence too. It’s because that area around my house was different from others.”
Tifa: “Such as the flowers being able to bloom?”
Aerith: “Yup. I think that’s the same way my mother, Ifalna, was able to come over to my room. But I can’t even aim to try and do that. It’s because I’m not a pure-blooded Cetra, and more importantly, I don’t have the same state of mind as when I was a child. I think that’s a huge factor. I can no longer tame that pure-like feeling similar to when I was a child. I wondered if I became insensitive once I grew up.”
YMMV may vary, but I find this a much better, simpler, and more elegant explanation for her waning powers than Whispers, touches, defeat of Harbingers etc.... She's only half-Cetra, she grew up and lost her childish simplicity, and she needs close proximity to fertile land. The huge concentration of Cetra memories and "power" at the Temple of the Ancients will compensate for these deficits. However, I imagine she'll be able to access her powers whenever the plot requires her to do and there will always be Reasons.
It's possible, isn't it, that it was the Whispers giving her the visions?