Suppose you have been wrongly accused of murder. You know you are innocent but
Given that I'm in the rotten situation of being charged with a murder I didn't commit, it may be a good thing for me that I can avoid being executed. But I'm in a rotten situation; all things considered, my situation is bad. It's bad even if I would be a fool not to accept the plea deal. It's bad even if the possibility of plea deals is a good thing in general, and even if the prosecutor is acting entirely responsibly. (Let's suppose the murder I'm accused of is a particularly heinous crime and that the evidence is strong, even if it points to the wrong conclusion.) It could be good for the justice system overall that plea deals like this are possible, good for me from a narrow point of view that I can accept the deal, and yet, given that I'm innocent, a bad situation from my larger point of view.
What about coercion? If we use that word, most people would understand it to mean that the prosecutor shouldn't be putting me in this situation---that a responsible prosecutor wouldn't offer such a stark choice. That's a different question from whether the deal is a good one for me from one point of view or another. Though I'm innocent, the prosecutor might have strong reasons to think I'm not, and she might be following reasonable guidelines scrupulously. That would make the word "coercion" a poor fit. On the contrary, it could be that the prosecutor firmly believes that offering this deal has an element of mercy about it and that she privately hopes I'll accept it because though she thinks I'm guilty, she abhors the death penalty. Whether the deal is coercive is a question about whether the prosecutor is doing something wrong by offering it, and not simply a question about how things look from my own point of view.
Also: whether it's better for me to have the plea option than not to and whether it's coercive aren't the same question. Leave your specific example aside. Suppose we have a case where a reasonable punishment would be, say, 5 years. Suppose I'm innocent and the evidence is not overwhelming, but the prosecutor needs more successful prosecutions under his belt to keep the voters happy. Furthermore, the prosecutor doesn't like me. Suppose the deal I'm offered is that I get 8 years if I plead guilty and 20 years if I lose at trial. It might be that all things considered, I should accept the deal. But given the particulars, it sounds like coercion. The prosecutor is putting unreasonable pressure on me to accept a bad deal just because it suit his purposes.
There's no puzzle here, and this brings us to the final point. The thing about coercion is that it works by putting people in a situation where their options are wrongly narrowed and where given those bad options they are rational to accept a result that in a just world they wouldn't have been faced with. In the case we just described, it's better for me to have the unreasonable option of 8 years in jail than not to. But I'm still being coerced.
So maybe your case is coercion, and maybe it's not. That's a different question from whether the possibility of plea deals is good overall, and it's a different question from whether you are wise, all things considered to accept the deal.