I disagree that it is at all likely. It would take months if not years worth of launches to fill an orbital shell with debris and nobody's going to do it before

I disagree that it is at all likely. It would take months if not years worth of launches to fill an orbital shell with debris and nobody's going to do it before starting a war. It's practically a declaration of war on its own. It won't happen at the beginning of a war because nobody starts a war with the intention of having it drag on for years. If you're winning a war you're not going to do it and if you're losing a war you're not going to have the resources or time to do it nor the ability to do the launches without getting your launch pads destroyed. Furthermore it doesn't shut down all access to space, as you can launch through the cloud to a higher orbit.