I disagree. While only the first portion of the buff is to alert you of wards, it is also most effectively for the point of revealing invisible wards. If that function is undermined because of a pre-existing Blue Ward that is obviously seen and not remotely concealed in any way, then isn't the most important function being cut-off for the wrong reason?
I understand what you're saying and in a lot of cases it makes sense. For example, if someone places a Blue Ward in the corner of Dragon's crater where you have no vision without a ward or being there physically, then Raptor Buff is functioning as intended when it alerts you to a ward's presence. However, if there is a Blue Ward out in the open and you already have vision of it, then why should the second-portion of Raptor's Buff begin expiring?
You say that it's linear, simple and clear, but I do not believe that is acceptable enough in this case. Complexity is necessary in order to preserve important details. So no, I do not believe someone dropping a ward on you under your turret is cause for expiring Raptor's buff. There is no reason for the second portion of the buff to proc simply because a ward of any kind is nearby. It needs to be invisible or at the very least unseen (such as being in brush or well within the Fog of War).
I view this as an issue that needs clarification or fixing, not as a can of worms. I don't see the fear of complication in making the second-portion of Raptor's Buff only activate when there are concealed or invisible wards.
Edit: Once again, at least have the dignity to respond if you downvote, dear Internet Lurkers!!