The win rate you should be comparing to is not 50%, but the average win rate in the tier you’re presenting. You need to be close to that average win rate in order to stay in the tier, and if you’re below it then you’re losing SR.
In GM, where Mercy’s win rate was 50.77%, the average win rate is about 54.3%. That’s far enough below the average so that If you’re playing Mercy in GM then you’re throwing. Moira’s win rate this week is a bit of an outlier in GM. She’s usually at about 52.5. No other healer has a problem this big there.
In Master, the average win rate is 52.5. Again, no other healer is even close to being as far as Mercy from the average win rate. In diamond the average win rate is about 51.5%.
A difference of 3-4% from the average win rate is huge. The spread of all win rates is about 6-7%. 3-4% is worse than the situation Ana was in prior to the changes. It’s the worst situation a support has been in in higher ranks in the past 6 months (that’s the depth of the Overbuff database).
I recently posted 2 posts about this. One giving the weekly averages (both win and pick rate) for Mercy since her nerf in ranks diamond-gm and the other explaining the statistical levels of confidence we have in that data. I’m linking these in case anyone’s interested.