It is a interesting problem, but it cannot be solved by marketing, classes, etc. It your suggesting limiting acceptance to only those who will continue to dive, based on a formula or a credit rating you will doom the sport.
What we are truly talking about is interest and acceptance into the sport by... well us. If you really think about it, how many other sports REQUIRE you to have a buddy? How many sports do you actually depend on that buddy for your life should you need too? I live in Florida, and I have the priviledge to be able to dive whenever I feel like it. The Gulf too cold? I can go over to the east coast. The weather is bad, waves, chop and the boats don't run, I can go to the springs.
Guys this is not about bringing people into the sport, marketing accomplishes that. This is about embracing a newbie, encouraging him to be a excellent dive buddie. Make sure the new guy has someone to dive with, work with them. Teach them how to be a "hole in the water", help them do it right! Fear kills our sport more than anything, a bad experience, lack of someone to be there for them kills it all. I am not talking about chest thumping I can dive this deep and that long, I am talking about ending your dive without complaint because you newer diver sucked his tank dry. Show them, help them how to conserver air, what to look for underwater. If a you as a exp diver alienates a fellow diver because his skill level is below yours, you helping drive them out of the sport.
My wife and I embrace newbies, invite them to come along. Show them how to buy used gear and what to watch out for, don't make my mistakes. I have some of the best friends now that are newbies, they will never catch me in logged dives because they dive with me and I dive with them.
My wife is my best dive buddie, she got me into this sport and she shakes her head in amazement on how dedicated I am about it.
Start a mentoring program at your LDS, make sure that any newbie has a exp dive buddie if they need one. Invite people along, show enthusiasm when you see people attending OW. The ones that stay with the sport are the people who are buying equipment and helping the LDS maintain good pricing.
I was at a party at my wife's work in Miami last year, and everyone I talked to was a certified dive, they have all not dove in years! The live within 45 minutes of Key Largo and one hour from West Palm and they don't dive.
Me? I was lucky, I met a bunch of cave divers and they raised me as one of thier own.

I am not cert'd for cave and not sure I ever will, but they have been where I am now and I can call on anyone of them for advice or a beer.
It's not interest we need, it's encouragement.