For this, I think you have to look at the philosophical make-up of Nemesis.  It was her job to take entities down a peg.  She was/is retribution for hubris.  I think it's fair to say that the old pantheon as well as a significant number of other long ago Gods/ Goddesses would feel anger towards mortals (as an example) for the ease in which they turned away from them.  There is obvious hubris in that.    In truth, there has even been an active organization trying to assure that is the case ( The True Venatoris).  I don't find it any stretch of the imagination to assume that even though Nemesis might not be at risk of being forgotten in total personally...there aren't quite a few Gods, Goddesses, gods, and goddesses who are approaching the cusp of banishment.  Many more have already faced it.  So, I don't think Nemesis would have to be an "Outsider" at all to perform as is her nature...especially since she probably was well familiar with many of the Gods, Goddesses, gods and goddesses that no longer exist in reality because mortals no longer recognize them personally or historically.

It would also explain how this war with the "Outsiders" has been going on forever.

I've wondered about the loss of worship and the Oblivion aspects for a bit. Maeve brought up Atlantis which, depending on the legends, went under the waters 12,000 years ago. So the fae remember the lost city but the Archive doesn't acknowledge their memories as part of the aspect of mortal knowledge. But what happens if solid proof of Atlantis popped up at sites like Gobekli Tepe or Nabta Playa?  Does that restore erased aspects of the Archive?

On the worship front, I hold that people placing tattoos of ancient symbols counts towards energy of that pantheon. All those ankhs on people could be fueling an Egyptian pantheon revival. Isis may be returning.