As player and customers we might think that GW'd decisions are stupid on a day to day basis. All of them originate from a centralized motivation though... making money.
40k is what is making GW money. More than anything else they've ever sold. It's how they pay their bills and 40k sales would really need to drop into the sewer for them to do something drastic like they did with WFB. While GW sales have been terrible for a few years, 40k is the only bright spot. AoS was a recognition that the Fantasy was in the sewer, and they basically are throwing a hail mary to see if it works.