There’s really only one thing on anyone’s mind when talking about Pokémon GO right now, the arrival of Gen 2. It went from being one of my highest CP Pokémon, to not even in my top 15. So for 400 CP, that's 30 candy, which takes 150 kilometers of walking using the buddy system. 500 CP lost = 50,000 Stardust, or however much it took to upgrade him that much. This is why CP nerfs hit so hard, because everyone wants to be the highest run in the gym possible, as there is a lesser chance of being knocked out.

Source: Forbes February 19, 2017 19:29 UTC