@Seth McPherson, thank you for your comments. I am receptive to any counter-argument that can stand up to scrutiny, and I especially appreciate your long-term perspective of this tree. That is important to me too, and I intended to ask my arborist about this, but we got to talking about all kinds of things and I failed to come back to that. I wanted to ask him how the tree should be expected to respond to trimming 1/3 off this limb in the near term and long term, and what maintenance would be required in the future. I am expecting epicormic growth which will, of course, be weakly attached, but I don't know how much and how big and how it should, or should not, be pruned in the future and how often.
Please don't misinterpret my questions and comments as criticism of your perspective. I am not fighting you; I am examining your argument. I am the ignorant one here, and I asked what you experts would do because I don't know what is best for this tree. But to understand this, I need to turn your concerns back around to you regarding your advice to cable the limb and leave it as is. Using your words, that, too, would seem to drastically change, in one day, its shock-absorbing ability. And what would this tree look like in the long term assuming the cabling preserves the limb? Will that limb simply grow to the ridiculous point of dominating the whole tree as it seems to be doing now? I don't know. I'm just trying to picture how the tree and that limb will look 30 years from now, and how much more cabling it will need, or if the cabling will be effective, and what maintenance will be needed. I will be long dead and gone then, but I still want to give the tree its best chance at a healthy life beyond me.
By the way, everyone assumes there are no targets here, but there could be depending on timing. The tree is next to a driveway and near a sidewalk. There could be a car parked in the driveway, and the sidewalk is used by neighbors and several students who walk to/from the two schools one block away. The chances of hitting a target are small but possible.