I have been on the receiving end of bad rezzes before, and I have cringed watching Mercies dive deep out of position to save a lost cause, killing herself and maybe the rezzed target in the process.
But denying a teammate a bad play is not an ability that any other character has. If I’m on Zarya, it’s bad for me when Rein pops his shield up right when I was about to absorb some energy with my self-shield. As a squishy, it’s bad for me if Orisa repositions her shield just in time for me to catch a Storm Arrow with my face. As a Mercy, it’s bad for me if my teammates duck around a wall so I can’t GA to them. As anyone, it’s bad for me if my DPS shoot into a reflecting Genji and it kills me. As a main tank, it’s bad for me if I say, “Let’s go left,” and lead the charge going left, and then my team doesn’t follow me. It’s bad for me if a Lucio, Pharah, or Junkrat either boops away the headshot I was about to hit, or accidentally flings the hero into range to hit me. It’s bad for me if Lucio uses speed boost on me, or Mercy uses damage boost, when what I really need is a heal. It’s bad for me when our main tank initiates into a fight we can’t win, leaving me with the choice of either letting the tank die on their own (stagger), or rushing into a fight that I don’t think we can take.
All of those things can lead to me ending up in a bad position, getting staggered, or our whole team getting wiped, due to my teammates’ incompetence, but it doesn’t make sense to say that I should be able to disable the parts of their kit that would mess me over.
Rez is already constrained by a bunch of factors, the most obvious one being the teammate has to be dead and not about to respawn in the next 2 seconds. To make yourself literally unrezzable is not just making a shot call; it is entirely disabling an ability that Mercy would otherwise have.
And as often as bad Mercies attempt bad rezzes, there will be bad teammates who write off a fight far too early and refuse rezzes that could have turned the game.
In a team-based game, there is no way to prevent your teammates from making bad decisions that you have to suffer for. And while this sugestion would stop Mercy, specifically, from making this particular bad decision, it could also stop her from making good ones, because it’s literally just tying her hands entirely. I don’t think that’s a good mechanic to introduce.