The guys getting commands now have been with VS 7 to 8 years. The guys joining the company now will be nowhere near command when the current expansion ends - note the emphasis on current expansion, who knows what will happen in the future.
As to job security - as has been pointed out, VS has diversified its route structure quite a bit since 9/11, but I think that most of the profits still come from the US routes - though no doubt that will change with time as the new routes bed in. It's also been pointed out, by that fine scribe Scroggs, that VS is a private carrier and private carriers will never be as secure as state carriers. We're in good shape at the moment, and we're no longer the new kids on the block trying to find our feet, but if a general catastrophe hits aviation (maybe Bird Flu??) then we're not going to be immune. If job security is your overriding concern, then I think BA is a better bet.
Having said all that, this a good place to work - not perfect by any means, but the best job I've ever had. Just don't tell management I said that..