Some important characters are not treated as "deaD" if thewwy are that plot-relevant later, basically like when you have perma-death on in Fire-Emblem and plot-relevant charatcers don't "die" so they can continue on with the story. We can't cut off the entire fifth era because Pellaeon happens to die earlier in another place, nor the second one if Thrawn dies under the EotH. There's no way to make that kind of branching either make sense or function. Pellaeon isn't an era two hero though anyways, so I'm not sure how you'd have gotten him killed as the Empire in era 2.