Mmm. I think the invasion is meant as a symptom rather than an ultimate cause of Armageddon. Probably even the "loss of faith in Aslan" subplot is also meant as a symptom. But the thing is, those are not symptoms anywhere near dire enough to serve as warning signs for the end of an entire world, particularly not when similar things have happened before without horrific existential results. The Narnian world, to all appearances, is getting along just fine -- sure, Narnia-the-country is having a really bad year, but that's nothing new or different -- and then boom, lights out, the end. Lewis puts significant effort into creating an ominous, defeated, helpless atmosphere, but he never does overcome the scale mismatch. (At least for me, anyway.)