No, were not. That's my point, plague did not have a higher winrate because of the things the card does but because of how you play the card. The card either has huge impact (influences winrate in a positive way) or it sits in your hand which influences your winrate in a negative way without actually making the cards winrate worse because you didn't play it. At the same time the stats are very misleading because every time you play a 1/5 taunt for 6 mana against control it is obviously an insanely bad play. Winrates just can't represent a niche card like this at all.
That said, plague currently doesn't even have a good winrate when played. Funny enough, ultimate infestations winrate is about 20% better.
Compare the when-drawn/mulligan winrate with the played winrate. It's night and day. Pyroblast is probably the weakest card in the deck, yet it has the highest winrate. According to blizzard logic pyroblast needs a nerf.
We have never, and would never, make assumptions of a cards power level based on play/win rate. It is not a valuable metric for many of the reasons you already stated. We use a metric based on draw/win rate with some minor adjustments for the number of cards played that game. This has always been the metric we've used.
That's pretty much the same thing with slightly different flavor. In the end you used winrate (and that's what the post said, too). It doesn't matter if you slightly rig it, use on-play, on-draw or on-mulligan, as you said you can never properly evaluate a card like spreading plague that way. Yet that was what you used to determine which card needs to be nerfed, according to the blog post.
No matter how you spin the metric used, it just isn't applicable. Something silly like "we know UI is really strong right now but we just like how powerful it feels and wanted to tackle druid issues from another angle instead" would have been more acceptable of an explanation back then.
I really want to play mouse in those situations. What were the designers actually talking about? I just can't imagine that they really thought infestation is ok. I mean how many more expansions until it rotates out? You basically can not release a good druid card in the next year because due to the existence of infestation it will automatically make druid oppressive.
Part of the goal was to make Druid less powerful. Part of the goal was to make Druid feel less bad to play against. We thought nerfing plague did a better job at both of those things. I don't know what you mean when you say properly evaluate. Statistics don't do a great job of telling the story of how a card feels to be played against you but I can with certainty the card that leads to more Druid victories among SP and UI is still SP, even after the nerf.
Iksar
We have never, and would never, make assumptions of a cards power level based on play/win rate. It is not a valuable metric for many of the reasons you already stated. We use a metric based on draw/win rate with some minor adjustments for the number of cards played that game. This has always been the metric we've used.
Iksar
Part of the goal was to make Druid less powerful. Part of the goal was to make Druid feel less bad to play against. We thought nerfing plague did a better job at both of those things. I don't know what you mean when you say properly evaluate. Statistics don't do a great job of telling the story of how a card feels to be played against you but I can with certainty the card that leads to more Druid victories among SP and UI is still SP, even after the nerf.