Well it is probably the biggest game store on the planet. My experience with other independent retailers is the same though. From talking to them in person or at trade events they almost all have the same experience of their 40K and AoS client base.
With respect that's still not a representative sample. I think later on you say something about stepping out of our own shoes...

We're probably both expressing our own cultural/national experiences and biases. I've an Anglocentric view of the hobby, yours is (I'm guessing) North American.
I suspect that if there are any churn and burn kids they are probably going to be at GW stores since people with no experience of the hobby would go there directly. The amount of product available at the average GW outlet now is pretty small though so I don't know how much an impact that makes since anyone getting more involved in the hobby will have to buy somewhere other than a local GW location.
The type of hobby store you work in is actually atypical of independent stores in the UK (and, from my time working in GW not the majority worldwide either). Most GW products sold by independents in the UK go through toy stores, where GW stock sits alongside dolls, traditional board games Lego, teddy bears, train sets etc. Hobby stores like yours are infrequent here (though seeing a resurgence in recent years to my untrained eye). As far as I know toy sales are significant in NA and Euro markets as well, though smaller than in the UK.
Hobby stores, of the type run "by gamers for gamers" inevitably attract an older, dedicated crowd but they're not the majority of stores, and nor do they generate disproportionately high sales across the independent sales as a whole. They're important, and they provide great resources and support to the older hobby section but they're not the bulk of GW customers.
Many GW store managers in conversation always used to insist it was the veterans and regulars - older hobbyists like themselves - who kept the tills whirring. Analysis of actual sales figures and impartial observation always revealed it was the younger, passing through crowd who really kept them going. A good manager new the value of both.
I don't think that it is nostalgia as much as GW is working to bring back their Specialist Games range and their "non-core" titles as they have been asked for more than a decade.
Older guys asking to have back games they used to enjoy in their youth sounds like the definition of nostalgia to me.

They have these existing and established properties and it makes sense for them to once again expand them to help broaden their market. The fact that we have nostalgic feelings for them shouldn't be confused with GW marketing them as nostalgia titles. We all need to try to step out of our own shoes and look at these releases independent of our own presumptions about them.
To be fair I am basing my perceptions of this on places like here where the overwhelming feeling is "Cor, Necromunda's back - I used to love that!" Clearly it can't be only nostalgia that's driving sales, they must be attracting new players (or so my fat dividend cheques tell me

). Wonder if they'll stay or churn and burn? (My gut feeling 90% churn, 10%stay).
They are using them to get gamers into their marketspace in the way that the games were used, accidentally or deliberately, in the past. Necromunda has a totally different experience in the core boxed set than it did previously for instance. Its just that not a lot of companies would build a huge range of games like this and then ignore them for as long as GW has.
They didn't ignore them. A lot of time and money was invested in Specialist Games when they had their own team and magazines. And the sales didn't justify it. They needed time out of the limelight, time for some new Games Devs to be given their head to have another crack at them and, just maybe, time for a little nostalgia to kick in.