Pulling data from here;
https://www.overbuff.com/heroes
And looking at "last week", "competitive", and "all ranks", and the top winrate is Symmetra. Then Torb, then Junkrat, then Orisa, then
Zenyatta, and finally Mercy. And that generally holds true at most ranks; at all ranks from Platinum up, Zenyatta's got the higher win rate, not Mercy. At Grandmaster, Mercy's win rate craters, while Zenyatta's rises to #2.
If we look at tie rate, where if Mercy was that super-effective we
should expect to see her much higher, since two Mercy-running teams should stonewall each other, Mercy is
again not in the top 5.
The data isn't showing what you're claiming. She has a high pickrate, and that's it. And if you look at "last 6 months", Mercy's pickrate only drops from the current 16-17% down to 12.5%. She was ALWAYS stupidly popular. All that really happened is that she's seen a bit of a bump because she got some good changes; there's no evidence in the data here to suggest she's swinging matches as extremely as you're claiming she is.
That people are picking Mercy over Ana could simply be that Ana's been frustrating since her nerf (she was my first Golden Gun, and I barely ever play her now, I definitely prefer the new Mercy over her), and new Mercy is just more
fun. This is an issue of Ana being
meh, not Mercy being OP.
Ana's low win rate also isn't new; go to the last 6 months in the same link above, and Ana is 2nd from the bottom in that time frame. She's been performing poorly for
months, it isn't because of Mercy, it's just that some Ana hangers-on have given up because the new Mercy is more engaging.