I would love to see this diamond in person, but I can't see it being a good investment given the technological change coming, both in sorting for diamonds up to 5000 carats and the real risk someone will figure out how to grow diamonds this big.
However, I do think there will always be a market for large diamonds extracted from cold hard Earth. Yes, synthetics will likely eventually surpass these stones. Owning one of the largest natural stones ever found will probably always seem sexier in many (read: likely non-high-tech) circles of ridiculous wealth.
Production has risen steeply over history (http://www.gia.edu/cs/Satellite?blobcol=gfile&blobheader=app...) but "The total global production from antiquity to 2005 is estimated to be 4.5 billion carats". So for an all-time estimate, 11*360 + 4500000 / 360 = 16460.
This seems improbably high: surely at least some of those 16k giant stones would've survived? So maybe Lucera's current mine got lucky, or it's unique in providing lots of giant stones. ("It has an enviable share of the world’s heavyweight diamonds. Of the 52 biggest diamonds discovered in the past three centuries, 10 have come from Lucara in the past three years. With modifications under way at the mill, Lucara will soon be able to recover diamonds up to 3,000 carats, and next year, up to 5,000 carats—as big as a softball.") Still, I would personally stay the hell away from giant diamonds as any kind of investment or prestige trophy.
I wouldn't extend the ratio of large diamonds found at Karowe (or Letseng) across the whole industry. They are fairly unique mines.