Hi Sean, I would consider myself another 'in the know'
I am on the management team for the
Virtual Pilots Association, and would certainly say that for our site, interest in virtual airlines is not dying off. It is no doubt however true that there are plenty of virtual airlines out there that never seem to refresh, and I would hate to think what the average lifespan is for a virtual airline. And even a few larger ones even have gone by the wayside in recent years (The very large Flying Tigers Group comes to mind).
VPA however is still very busy and active, and has been since opening in 2004. We have a steady 500 member limit, and the waiting list to get in ranges up to a 3 month wait (90+ people on the list) but currently there is only 14 people on the waiting list. In Jan 2012 VPA pilots filed 6587 pireps, and a total of 12496 hours. (that's an average flight time of only 1.8 hours, and 25 hours per pilot on average). 8400 hours of those were non scheduled flights, or pure and simple GA flying. GA for us includes doing whatever you want in a jet airliner, or Cessna, or bizjet etc..
Most of the remainder was actual airline flights (3800 odd hours for any airline in the world) plus a bit of military flying was done by pilots.
So we have a bunch of very active pilots in there. And while we allow pilots to fly practically anything, you can see for the most part pilots would rather fly a bit of GA anyway. So we aren't all tubeliner pilots! (though I am lol)
As for your idea of starting a VA, I've seen a few come and go, and here are a couple of tips for success.
1. A GOOD website. You really are competing with some really great ones out there, what would make someone choose your one? Having an online presence (vatsim and ivao) is one of the best ways to showcase yourself.
2. Now you've got their attention, they need a reason to stay. Good fleet, good choices, good website features, and perhaps most importantly, something to do. It gets pretty boring just logging the same flights over and over again (if not simulating a real world airline), especially in a small country like NZ. Awards, promotions, goals, statistics, tours, missions etc.
Now a question from me...
For those of you who DON'T belong to a virtual airline, why not? Surely you must get bored just looping the loop and having little to inspire you to get on the sim and fly? Where do you find purpose in your simming?
This is where VPA is quite useful I believe. You can fly a 747 on a scheduled airline flight one day, and yet still muck around and fly your Spitfire round the clouds the next day. Or just spend the next 5 years mucking around in helicopters, spitfires and C152s doing circuits. VPA accepts it all. It's nice to have a way to keep a record of all this flying, and not being tied down to schedules, (or out of date schedules). I certainly would have quit the hobby years ago had I not had some sort of goal system, or purpose to my simming, naturally provided by a virtual airline. They are really neat things in my opinion.
I belong to another VA (pilot only) that is also ticking along nicely with many members and many flights per month.
http://www.phoenixva.org Also another fine VA.
(sorry for the length)
Posted: Thu Mar 15, 2012 1:46 pm
but as Ian says we do have our very own Virtual Air New Zealand which you may be interested in. We run a realistic operation with a career based system, pilots start out on the Beech and end up on the 777/200-300 and 747-400 as they accumulate hours. We have regularly updated real-world ANZ schedules, a members forum, Moodle training system, automated PIREP system, and training captains for each of the aircraft should you require assistance. We are also an official VATNZ VA partner as many of our members fly online with VATSIM (although thats not compulsory)
.. and their mistakes ... look what happens to the real airlines 
........ 
"I am a fighter and not a lover part" 
... 
and a TUI cheers to you to