I think there is some confusion between what is being suggested here. Replacing "get there" with "arrive" was a red herring. The important distinction that cathjohnson made, which I agree with, is that we don't say "We [arrive|get there]...", we say "We'll [arrive|get there]...". The difference is between present the simple future.
Nope, I totally disagree. You would never get a native saying 'we arrive when we arrive' in normal circumstances. 'We get there when we get there' is perhaps a 'stock phrase' to indicate the person doesn't know what time they'll arrive, although it would be far more natural to say 'we'll get there when we get there'. If you listen carefully, you'll hear this 'll all over the place among native speakers. 'We arrive when we arrive' is just too weird!