Well I think there's a bit going on here:
A bit of background: I actually applied to OnDeck, and spoke to their team, was about to do one of their programs and decided to not do it at the last minute. Know several folks in the OnDeck team well, as well as more than enough people that have gone through their program (including advised them in specific circumstances to actually go through it).
OnDeck definitely started as a good initiative and definitely is valuable. It's not about "meeting interesting people", but more like "become part of a club". Now the latter is different because its the same as joining YC or a university. The "education" is important, but often not the most important part. It's like come for the content, stay for the network.
The content, I am sure one can get elsewhere, but the network if done right is great. For folks starting out, or who don't have exposure about a relevant program (say Chief of Staff, or Founders), this is actually great and worth the price. Secondly, for a one time fee, you get to become part of the network for life (again, similar to things like YC, universities), and so that is definitely valuable.
Whether that can be replicated in other ways, and can one do it manually? Ofcourse. But you can see how it takes longer, because you'd have to "prove" to the other person that connecting with you is worth their time as opposed to two Ondeck Alums catching up wthout any agenda coz of the pre-assigned credibility of them being part of the same program.
The bigger challenges with programs like OnDeck is, does the experience of the alums change as the program scales. Coz there's definitely enough people from the first year or so, who will vouch for the value the program provides (I personally know atleast 50+ such people). The issue is whether that value trickles down to people who will join today. It's the same argument around, does it make sense for someone to join YC today.
And the answer to that is tricky.
If you already have a network and are doing something the second time, then you probably don't need it. Like if you're a second time founder, having raised money earlier, you don't really need YC or OnDeck or anything of that sort. But if you're an engineer who've got NO idea of how startups work, OnDeck is a great place to find your first few customers, get your first few investors, be part of a community and have a directory of people globally that you can reach out to.
I'd have loved to see this model evolve, esp if they figure out how to maintain that "closeness" within the alums as the years go on.