Warhammer had 35 years of background development, AoS has 10 months
That's a bit like Gordon Brown claiming that he'd only been Prime Minister for a few days after he'd taken over from Tony Blair in 2007 during questioning in the House of Commons
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GW have had the opportunity to plan for Age of Sigmar. They could have chosen to move the Old World forward by introducing new characters and advance the time line to accompany the new rules, rather than just rip everything up completely. Having chosen to have something that is, to all intents and purposes, completely new, they needed to provide more of a gripping narrative to engage players, in my opinion.
There are some parallels to what happened when Epic Space Marine/Titan Legions was replaced by Epic 40K. The idea was a good one, but the streamlining went too far and the marketing and presenting explaining the reasons for the change and the new approach were poorly executed. My impression is that this is what has happened with Age of Sigmar.
Again, I stress that I'm not knocking the idea of making the change. Warhammer was in a very poor state in its final years, with horrendously bloated rules, units which were too large, and serious balance issues. I am just unconvinced that the way in which the change has been handled has given Age of Sigmar a high probability of success.