...Still, 2nd 40k edition sounds great!
2nd ed was verifiably great. Not perfect, but a very good, fun, and complex game.
Forgive me if I sound at all jaded but...wouldn't any other company I don't know...maybe have released a blanket question perhaps of "Why do you not play WHFB?" or "What is wrong with WHFB?" that their fans can reply too.
I think you might be missing the point of marketing and market research. In this case GW knew that fantasy wasn't selling. And I suspect they had ideas as to why, but knowing that they don't need to survey the existing customers they needed to survey prospective customers.
My understanding is that gaming communities have a demographic breakdown including some veterans and then lots of new players who will buy an army or two or maybe just a box of figures and some paints. Veterans might organize events and paint beautiful models but it's the new players who drive the bottom line. It's the new players in the first rush of aspirational buying (dreaming of, "oh boy these space marines are badass!") that make the big purchases.
GW needed to survey those people (and their parents.) And seemingly they have and have released a game for them.
...What's the lowest point level you have seen people play fantasy; whats the lowest point level you have seen people play 40k? The games are only getting bigger. I can't think of a new player in my FLGS's 40k group that doesn't have a parent involved as well, and that has been years ago at this point.
GW has been inflating the size of the game since the switch to 3rd ed 40k where they cut points costs in half. But I bet they found out it was a double edged sword. It means each new player earns GW twice as much as they once did, BUT there's a diminishing return. The higher price prevents some people from buying in at all.
Age of Sigmar seemingly solves this by drastically reducing the barrier to entry and simultaneously lifting the top end too - because you can use whatever you want.
I think the real question is once we know if AoS is successful or not, how will that business model change the future plans for 40K.