Picks always drop. That's become the norm. This year looks less than recent years.
Carlton not being willing to trade 12, but offering future first and second for 13 shows how they rate this draft.
Sydney's first round pick and 22, which is an end of first round pick, may very well get you two kids that would be rated very firmly in the first round in most draft years.
If the draft is as deep as they say and outside say the top half dozen pretty even for the next 15 to 20 then Sydney's offer makes less sense. You do that type of deal when the draft pool is shallow.
I'd rather hold the picks they have than do a 2 for 1 exchange with GC.