@ETPro When I read the way you explain it, with centrifugal force flattening a galaxy, it makes me think you are conceiving of it in a way that I don’t think is accurate. What I think might be accurate, and what might be at work in solar system formation, is that a concentration of mass gathered more mass together in the same plane, due to gravitation with itself, inertia, and collisions cancelling opposite momentum. But I don’t think that centrifugal force (which is simply the effect of gravity having a net force towards the center of galactic mass, and not in the direction of the stars’ motion) would have a flattening effect. What might would be a concentration of mass in the plane, simply because the mass started out that way due to whatever orientation existed before.
Of course, in the theories I mentioned above, even that is not what is going on. In the density wave theory, the spiral shape indicates the positions of density waves starting new bright star formation, rather than concentrations of where the mass is. Meanwhile, in the theory published two months ago, the arms are actually sort of being launched out of the galactic core.