If Mercy rezzes far in the backline of course she is save I don´t see a problem here - and she´s also not healing the rest of the team currently fighting the enemy team. Keeping her safe with a shield etc is only possible with good team work. So what´s wrong with her being safe when the team protects her? The team (or just one team mate) has to babysit Mercy for the 2 seconds and can´t do anything else. Those two seconds can be enough for securing kills because one main healer is not healing anymore. Fair trade. Not healing can get more than one team mate killed while getting just one back.
Yeah I admit I was wrong there. The description on the official Wiki page is confusing.
But to solve that problem: kill the healer first and/or seperate her from the team. Good players can do that, especially flankers. Mercy´s not OP with her healing.
Can´t say anything about Moira but I guess a very good Moira is still better than a Mercy in higher elos because of her burst heal potential.
You make her sound OP which she isn´t. Mercy is only good with a good team - her GA is useless when the rest of the team is everywhere but on the point. Her mobility relies on other players in the range of GA, it´s not like she can just teleport around the map.
Her healing can be outdamaged easily if the enemy team works together because she can (outside of Valk) only heal one at a time and even with Valk, a burst damage Ult and she´s alone. Or focus fire one hero during Valk, it helps too because she doesn´t get more healing. Just 60 HPS.
I´ve had my team die while in Valk without an enemy ult. I got killed mid-flight while using GA. Three body shots of McCree and Mercy is dead. I had team mates die although I heal them.
With a good team, Mercy is a very good healer, yes. But she is NOT OP. Choosing the best healer for your team is based on the situation, e.g. I switched from playing Mercy to Moira on attack because I can keep my team alive MUCH better, and I choose Mercy for defending because of her Ult to stall the enemy team from taking over the point/pushing the payload.