No this isn't about not getting everything at once, its about the fact they'd be a pair. It'd be like only showing Froakie but not Chespin or Fennekin till months later. Starters are a trio, eeveelutions are duos.
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So what if there's only 1 new Eeveelution? Why do you all think you are owed 2 Eeveelutions just because it was that was in gens 2 and 4? They don't have to follow a pattern, just like there was no 3rd version for BW, no RS remakes in gen 5, or how the first gym leader in BW weren't rock types. Get over yourselves, you shouldn't expect something just because it happened in a previous generation.