I agree with your end recommendation, but not with your logic.
Toyota didn't backspec and that is a binary yes/no fact. Just because you feel it to be true doesn't make it true.
it's nothing about the "shape" or condition of the engine, but the engine design. The Older engines, even if brand new consumed more fuel, and had dumber vvti and all that other fancy stuff, which means more combustion byproducts into the oil. OCI recommendation on the older cars is 7500.
That being said, all those engines are bulletproof, so the engine will be "fine" and will keep running, even if you threw sludge into a 1UZ or 2JZ, the engine is still going to run on the sludge, perhaps less than optimally, but will get you from point a to point b, and will run fine again once you finally change the sludge out. But end of the story, it's not recommended by toyota, and they did not say those words.