I feel like Fantasy and 40k always fulfilled different needs. Not just in regards to fluff, but in regards to gameplay. The ranked units, limited LoS, limited mobility, and weak shooting of Fantasy made it much more of a chess-like, geometric game of positioning. The shooting strength, rapid movement, and unlimited LoS of 40k made it much more of a fluid, dynamic game. The switch from Fantasy to AoS was jarring because it disrupted everything that made Fantasy Warhammer Fantasy: the core gameplay AND the narrative fluff. Many folks (including myself) weren't interested in AoS because it didn't touch anything that initially interested us in Fantasy.
I suspect the "AoS"ing of 40k will be different, as they're neither changing the core narrative, nor the core style of play. Its just a streamlining that brings 40k more in line with modern war games. It's still in the same world and it's still a skirmish game. It seems like a brilliant move.
Will it bring in a flood of AoS players? I suspect not. Some folks will play both, and it will definitely bring some additional 40k players to AoS as an adjunct to 40k. Now, though, I feel like GW might have two games that appeal to the same game experience with different skins. Personally, I appreciate a variety of different game experiences, and am thankful that there's so much variety available to gamers. Whatever floats your boat, you've got options!