Another ex-40K player here (I played this game exclusively for 5 years) with a huge and now totally inactive Tau-Necron 40K collection that involved many happy hours of building/painting and table-bashing. I've also noticed a big decline in 40k players at some recent club meets I have attended so can definitely see your concern. I bailed out during Fifth Edition for various reasons I won't go in to here, but having said that I really loved 40K pre-Fifth Ed; but the times they changed and new games took over.
As to meeting your challenge Brayo puts it well...
Hang out in gw stores and get them while they're young, build from there. Then once you're too large, i.e. players, then move to clubs.
I am pretty sure that the GW stores still do games nights Fridays and these are perfect venues to harvest and nurture young players. So Concept 1
: if you and/or your mates can get along to these evenings with a recruiting plan and a vision of 40K gaming utopia to infect the young players with then you might do well over time.
When I went to these games events there were models I could borrow at the store so I could have a crack with whatever was in the collection without the need to spend my limited $$$. Cost is a big issue for many young adults as they commonly don't have much cash and 40K has a pretty hefty entry level price tag, especially under the current terms of trade which exclude our access to models at UK/Eurpoean prices. So perhaps as Concept 2
you might want to look at getting a club-based model library together and make this available to new players (+/- small access fee if you want to build a fund to fix/replace any damaged models or make additions to the library); clubs have terrain collections so why not a model collection too?
might include show-case games held during school holidays at places accessed by kids (I'm thinking shopping centres and the like). I remember being impressed by show-case model railway displays at the Royal Show and toy-soldier displays at various shopping centers when I was a kid which went a long way towards cultivating my current list of polystyrene/white-metal addictions. Maybe those days are past but I reckon this might be worth a try if the right people are prepared to run it.
Campaigns - yes, that is a really fabulous idea. The best fun I ever had with 40K was in an escalation campaign with six players run by Baz. It was awesome but fell apart after life intervened and most of the players moved on. I was sad we did not finish it. This concept is a real winner but as Baz says you and all the participants need to be really committed to run a Campaign; I'd suggest you need to be reasonably time-bound and scheduled as well to avoid the 'life intervenes' factor!
Lastly, avoid the temptation to overpower your lists as your opponent must be given a viable chance to win; play with background lists not uber-lists in social games, particularly with new players. Also, if it looks like your going to smash your opponent throttle it back a bit to make the game closer (you'll be more likely to get a return game if your opponent isn't left feeling like they had their a... handed to them). I have made this kind of mistake before and it is not a good one.
Anyway Outcast, good luck and I hope you have a great result. It will require a lot of work but it sounds like you are up for that. I'm unlikely to join in at this stage but never say never, right?