Thinking about this from the Biblical perspective, if those who don't forgive others or let go of their grudges, are not forgiven by god... why should people forgive them either? This is result of Loophole Abuse and Immediate Self-Contradiction, because if you do somehow forgive them, yet believe god still won't, you are essentially arguing you are more moral than the highest authority on morality. The teaching collapses into itself.
I was going to add an example about how Naruto zig-zags this trope, with Sasuke refusing (for most of the series) to let go of his hatred for Konoha because of what they did to his clan, while Naruto refuses to ever give up on him and constantly insists that he can be redeemed, except I'm not sure where exactly the series ends up on the issue. Sasuke eventually gives up on his revenge and becomes a Karma Houdini, basically proving Naruto right, but what view does the series take on whether or not his grudge was justified (most people other than Naruto agree that he has to be stopped because he's a threat, but I can't remember what the series' consensus was on the validity of his grudge). Sure the FANBASE think he's a petty emo prick who needs to get the fuck over himself, but how does the manga play it?
Always expect the worst and you can only be pleasantly surprised.
I don't like that the description of the trail implies that not forgiving is not the right thing to do.