I also don't believe in 9th Age long term future. When people talk about new players, it's more about veterans from WFB trying the game system to use their favorite army taking dust on their shelves and expanding their armies.
True new players, who never played WFB, aren't that many for the 9th Age, IMHO. The reason is because this is not a game made for newbies, but people actually knowing a bit about wargames - and more specifically WFB.
Also, though some companies are trying to make miniatures for the 9th Age rules, I suspect it will never be on the same level than GW.
But that's just my opinion. Maybe I will be proved wrong, so far the community is quite active. And that's good.
The General's Handbook for Age of Sigmar is indeed a really good move from GW. BUt that's not the only thing; they also released starter armies at quite a good price and several small boxes to begin the collection at different levels. Choice is very important and since GW stores are quite the main window of the GW Hobby, it will help them to have more regular sales.
Sure, having veteran players to introduce new ones to the game is good, but if GW made things right, they don't really need the old players from WFB back. If the new blood is here in good numbers, it will just bring another new generation who doesn't get the hate from GW's previous acts. After all, the best and fastest way to accept new things is never to have known the old ones. That's why young ones adapt more quickly than old people.
I'm expecting more players will come to AoS. The key to success, to me, is to bring true new blood - and not just the old players who knew WFB. That's how GW will truly begin to innovate - when they stop trying to please the old grumpy players who can't accept changes and feed on nostalgia like Nurgle flies. That's how we got Fyreslayers, who are basically old Slayer dwarves with helms when they could have gone the full "fire dwarves" way instead. But no, they went the "Slayer" way. Booooring.
See the Sylvaneth and new Alarielle. Now it has nothing to do with wood elves. It's a true new faction with their own Goddess. And it brought much more enthousiasm than Fyreslayers, who just felt cheap with making new with boring old stuff we already know. Why blowing up the Warhammer world if it was to have the same units in the end? Just bring something new! Show us Metal Duardins! Or Shadow Aelves! Or even Female Orruks! Now that would be new!