The answer to this question can be found by looking at what folks were doing on the classic private servers once they'd cleared all available content. They either level a new toon, strut around faction capitals in "Look at muh gear" mode, or quit playing having gotten exactly what they came for. I mean, what was the original motivation behind the clamouring for officially supported pristine vanilla servers? Surely it wasn't backed by a fervent hope of something more that what vanilla actually was, was it?