I believe all of it. I've been learning about the effects of carbonated drinks and coke and the combination as coke drinks on the body since I was 20. Even back then they taught in med school the corrosive effect coke drink and also some brands of tomato sauce but not the leading one, were causing problems at that time. I've always been a bit outspoken about what I think of coke. I did drink it once too.
I too had excellent bone density in mid life.
Western diet women have massive incidence of osteoporosis by the mid 60's primarily because out diet is so acid producing and well coke is a fast way to add more acid to the body. We leach calcium from the bones to nurtralise the excess acid in our blood stream so our heart can function effectively if out diet is acid producing. That is the body's way, to care for the essential functions first and the skeleton second.
So a long time history of drawing calcium from the bone to keep the blood at the correct pH takes its tole over time whether this is trough not eating enough vegetables, excess consumption of animal foods, or though coke drinks, it doesn't matter. It isn't always about just taking in the minerals in our diet it is getting and keeping the minerals in our bones and that is done by keeping the blood at the right pH balance which is done though a balanced and not excessively acidic diet. So you are right, coke on it's own does not cause osteoporosis it is simply one big contributor to it in that it does acidify the blood and the body has to leach calcium from the bones to compensate.
Even a cup of coffee requires 35 mg of calcium to balance the blood pH due to it producing an acidic reaction in the body. That is mild compared to a carbonated coke drink's effect however.