Mercy sucks because she invalidates the other healers and the meta is almost exclusively focused around her resurrect which can completely save a game for a team over and over again.
You can play another healer....but only if you already have a Mercy.
Because this is a totally different situation than with Lucio, the only healer that can provide a speed boost, or Ana who could heal up a tank from near dead to full in 2 seconds.
That being said, they had to nerf Lucio like 4 or 5 times(including a slight rework) to get him somewhat under control, and the same to Ana. They've also come out and said that Mercy isn't in that good a position right now, and that something probably needs to be done about her current ult(not fun to hide when near full to pop res, not fun to an amazing team play being negated by one Q press), so we'll see what they do with her.
Ultimately healing is really hard to balance in the game. For them to change her Ult to something else, they'd probably NEED for her to be the strongest single target healer in the game without question since she has nothing else and has to maintain a really close distance to what she's healing. "She's easy to play" because she has nothing else in her kit, unlike the other healers.