Good discussion.
I come from a "Positive Climb" company and yes, the call does get missed sometimes but the HP just ends up calling "Gear Up" at which point the NHP realises he's missed the call, checks that we're going up and repeats "Gear Up?" back to the HP and if no objection occurs, raises the gear.
I think that the most important use of the "Positive Climb" call is in a low go-around (think catIIIa or catIIIb). It's pretty vital that you don't land having raised the gear: a missed approach initiated from 15' is likely to lead to a 'touch-and-go' in most passenger aircraft during normal operation.
In the big, long thing I fly you can have the nose quite a distance into the air, some time after rotation, with the main gear still on the ground, so the visual clues are not brilliant. However, there's not normally any reason to 'rush' the gear retraction, so why be trigger happy? I instigate my call from an increasing rad alt.
[Edited to change 'rate' to 'climb'. Too much flying has addled the brain
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