Do players who have 150 Pokemon say "yeah, I have 150, but I don't have Mew?" Of course. Those players are likely aware, though, that obtaining a Mew is not necessary to receive a Diploma/get a star on their Trainer Card in terms of Pokedex completion, and want one simply because the challenge they set for themselves involved catching 151, Mew included.
Yeah, I'll probably just keep this until next year.
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But within the fandom and within the context of this discussion (which isn't just about the slogan but about how important dex completion is in general), its obvious most fans think of dex completion as a complete dex, not excluding events.
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