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Honestly I think GF will introduce a new mechanic for evolving to Sylveon.
If it were to level up with Pokemon-Amie, that would mean they would need to incorporate Pokemon-Amie into every single future title.
Unless of course Pokemon-Amie just does something to the Pokemon, like raises Happiness or Friendliness, which can achieved by other methods in future titles.
But I don't think it would, or should, be 100% reliant on Amie.