His argument wasn't that Kadala wasn't useful, just that the rate at which legendaries dropped has decreased. It has, and by a good amount. I have made around 100 million gold selling items from Kadala, just to give you an idea of how many shards I have spent. And, since the anniversary buff was been removed (not saying that it had anything to do with Kadala, just that the patch that removed it was there I noticed this change), the amount of legendaries I have gotten from Kadala has gone down multiple orders of magnitude.
My own evidence says it hasn't changed. This is just conspiracy based off tiny sample sizes. It's RNG. I don't run into a dry spell and start suspecting a debuff, the same way I don't think there was an additional buff because it's raining legendaries for a couple days. This seems to be endemic among the D3 community lately. Pretty much Blizzard decides to make the 100% buff permanent and in response a portion of the community seems intent on complaining, day after day, about suspected undocumented debuffs in other areas?