What does shortening the wait time have to do with winning too many diamonds? Players can use diamonds if they don't want to wait. Players do as many as their troops allow, regardless of the wait times.
There is a finite list of fights, it's not indefinite so that if the wait time is shortened we can win hundreds and hundreds of fights, there are only so many to be won. It seems like a bad excuse to nerf yet another fun thing for the lesser fellowships. The top fellowships will stay on top and I applaud them. They will probably get a very scant amount more diamonds, the struggling fellowships will keep struggling and now get less.
For the majority of players, getting to the top is not a choice they make, many have 2 star troops of a basic variety. They don't have Frog Prince and Blossom Mage, along with high level AW's, and many don't have the max level Phoenix yet, so why are they the ones that need to have their small amounts of diamonds nerfed? They're not at the top because they can't get there yet, with or without a time reduction.