I'm only saying that a purely spend based threshold would be higher than $15K/year for Diamond. I would think that it would be less than $50K, but, then again, I don't think they should just give it to someone for claiming they will spend $50K.
Just some perspective, not me personally; but I know quite a few who spend in excess of $50K on 5-6 roundtrip tickets. (which can be as few as 10-12 segments, sometimes a few more...)
Look at the prices 1-2 weeks out (often even longer) for a rather short flight, in a market like JFK-AMS (nonstop) in business class. The prices can be staggering and approaching $6k to $8k roundtrip (same with the Boston market, nonstop).