That, however, begs the question what withered the warrior class... I went through the EB text files a while ago, and saw in passing a mention of Celtic (or Gallic, anyway) tactics and soldiery only becoming more sophisticated towards the end, in response to the rather pressing stimulus from invading Romans. I've no idea of the historicity of the postulation, but I understand the Gauls were among the more "advanced" Celtic peoples and the stimulus-response scheme certainly makes sense - if the old "charge and destroy" approach has been consistently trounced by another system as of late, taking a few pages of that system's book only makes sense right ?

After all, not long after the Conquistadors had finished dismantling the Native empires the weapon caches of prospective rebels started sporting pikes...

Of course whether any meaningful changes in doctrine can be managed in time, especially if they go against the grain of cultural inertia and established value-sets (and quite possibly require resources the social structure simply cannot provide), is a whole different question.