I think a lot of folks tend to overestimate this notion of "network" when it comes to b-school.
Keep in mind that it's not Skull & Bones, Finals Clubs, or some tight knit secret society.
The alumni 'network' really is just a very loose term for people who happened to have graduated from the same school, at various times in their history.
Even at the smallest schools, there are literally TENS OF THOUSANDS of living alums, and it's far from being your ten thousand closest friends.
Also, remember that as time goes by post-MBA, one's emotional attachment to the schools becomes more about personal nostalgia (not unlike the feeling you have for your high school right now) than some immediate, present asset for your career.
Of course it may not feel that way because b-school is front-and-center in your life now, but it's not going to be that way down the road.
Put it this way. If you're some random Stanford GSB alum, you can't just call up Phil Knight or Scott McNealy, whether you're some random 20-something kid, or a 50-year old. If you're a Wharton alum, you can't just call up Donald Trump, or if you're at Columbia, you can't just call up Warren Buffet asking for a job or to pitch a biz opportunity. Or at HBS, you can't just call up the partners at KKR and expect them to do much for you other than "hello back at ya!".
Also, if you're in your late 30s and 40s, where do you think your primary circle of contacts will be? Are you going to be trolling a b-school alumni database for leads? Whether you're looking for a job, or looking to raise money for a company, wouldn't you have enough contacts from the people you've known in the last 5-10 years (read: those people you've worked with in your industry, people you know from your extracurriculars, fellow parents you know from your kid's school, etc). Wouldn't that be where you spend your efforts? Would you really need to go waayyyyy back to your b-school database and contact strangers? Wouldn't that seem a bit desperate ("Hi, my name is PennTeller, I'm a fellow alum! I graduated 10 years ago, we never met or even graduated in the same generation. Meet me.").
Your primary and most useful network are going to be the people you've met and worked with within the last 5 years or so. If you haven't spoken much to them beyond that, they're just a few steps from being a stranger.
So, your b-school "network" are your FRIENDS that you end up hanging out with at school. Not even your classmates that you barely knew, once you're a few years out.
And you have no way of knowing where your friends from b-school (whether at your own school, or at other schools) will turn out. With 5-10 years, everyone ends up all over the place, all over the world. They will be in a random hodgepodge of industries -- if let's say you're still in finance and your ice hockey buddy from b-school is now happily settled in Nebraska owning a sustainable farm, he's just going to be a great friend, and chances are when you get together, you're not going to be talking about business. Maybe in your first 1-2 years out of school where everything is new, but no matter how ambitious one is career-wise, no one yaps on and on about their careers to their friends unless they want to bore them to death (or unless they are recent b-school grads).
The b-school "network" can be useful while you're a student -- but it's less about calling up random alums, and more about your fellow classmates. A lot of job leads come from your fellow classmates.
So let me ask you: given where you're at in your career, do you really think you're going to get a lot of great job leads from classmates who are not as far along in their careers as you are?
Again, your sentiment about "I'm far along in my career, but I want to go back for the network" is overestimating the value of the network at best. It's kind of like being a college grad and wanting to go back to re-do high school at Andover or Exeter to "network" with the rich kids.
One last time: no matter how "loyal" the alums are, it's not Skull & Bones. It's not the Marines, or some international brotherhood or world order where "we take care of our own". If you want that kind of kinship or "network" where members really take care of their own even if they're strangers to one another, join a church, or become a Scientologist. It's not some lifelong job/money tree that you can draw on time and again.