Delta's introductory email about the $250K Amex threshold said, "Because we want to ensure our most well-traveled Members can take full advantage of all the benefits of Diamond, a change is being made to how you qualify for 2019 Diamond Medallion Status." The implication seemed to be that Diamond would be a more elite group.

The Points Guy, among others, agreed: "At the heart of the airline’s decision is a desire to make sure that only its most valuable customers — those who spend the most money with the airline — are the ones that most often secure complimentary upgrades to premium cabins. TPG Senior Points and Miles Contributor Richard Kerr summarized why this move is actually positive for Delta's most loyal Diamond Medallions: “Diamonds in 2018, who earn either the $250k waiver or $15,000 MQDs, will be [a smaller] minority and rewarded more for their monetary loyalty to Delta. Thinning the herd to reward only those that earn you more money is a right business decision.”"

As far as exactly how much "thinning" actually happened, I have no idea. But it sure stands to reason that eliminating Diamonds who qualified under the $25K Amex waiver would have resulted in a decrease, not an increase, in Diamonds.