I don't think the airline would know how much to offload until all the passenger bags have been checked in, say, an hour before departure. The final weights would then need to be calculated with the cargo and fuel. By then, everyone would have gotten their boarding passes, so there is no way they can backtrack at that point to ask if they can be offloaded. The logistics of doing this, then retrieving their bags, would be atrocious, unless the airline starts denying random people boarding at the gate. That would also be a PR disaster.
Asking passengers at check-in whether they are willing to fly without their bags would also not be ideal. It is not a secret that flying a longer route would mean pulling some bags off, and this route was susceptible of this due to the known air restrictions. Terrorists may take advantage by sneaking in an explosive device or two knowing their checked bag may end up on some random plane other than their own. While this may be a far-fetched scenario as bombings tend to be specifically-targeted, I don't think it is in the interest of all the airlines involved when the passenger is fully informed before boarding that their bags won't fly with them, just in case they had bad intentions in mind.