Photos of the branches would be handy. It’s easier to tell if disease is affecting the tree by looking for dead leaders, feeble or lagging foliage etc…

That being said, this tree is vulnerable because of this damage to the truck, but it isn’t necessarily dying. Basically, a wound like that is a convenient way for bacteria or fungus to make its way into the heartwood. There’s not a lot you can do other than pray and enjoy what the tree has left. It might maintain vitality for years like this. If you start to see bark peeling off, branches dying, or leaves wilting, then there’s some kind of bacteria or fungus in the trees vascular system and it’s basically doomed.

If you replace it, don’t plant another apple there. It’ll quickly pick up any diseases the old apple had.