Delaware Mike -- is it possible that the functionality we want and the infrastructure you suggest are a matter of scale? I mean with membership at 100,000 does it start to make sense? Or is the threshold 200,000 members? I am not a tech person (I am the idiot they would ask to test it to see what kind of trouble I could get myself into

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Hi Jim.
First of all, there are those who are better at explaining this kind of stuff than I. My experience is from a WAN (Wide Area Network) viewpoint, and I have experience in voice and data CPE (customer premise equipment).
To a degree, yes, some scale comes into play. But the overall design needs to be looked at from a "systemic" approach. While a web service can be designed to function, traffic is still traffic, and a network of "good" design provides information in a timely fashion.
At a very high level, a lot of factors have to be considered for a high-traffic website. And that website has to respond well to high traffic demands, or someone like you and me

) ) will post on some board like this

) ) about how crappy that website is...

) ).
Looking from a WAN and CPE view, these things come with a cost. Add users and at some point additional bandwidth will be needed. Bandwidth utilization is always a on-going design concern, at some point a bigger pipe is needed. Add users and the number of concurrent data sessions on a router, switch, or server needs to be looked at too. Use too small a pipe to access the internet or a slow "box" to route or switch data and retries occur, data backs up, data gets dropped, users sit at their keyboards...banging on the Enter key or clicking away, and the whole mess might simply start to fall down upon itself.
The number of users is only part of the overall consideration too. How about applications, and how about the presentation of those applications? A "feature-rich" website sends more information downstream over those pipes than one sending only "stick-figures". Sure, a lot of that information can be cached at a user's PC, but not all of it.
Keeping our discuss at a high level, let's say DVC puts a lot of features on their website. Aside from the circuits and code to make the website function, there has to be sufficient interconnection between the web server and say, the server where my point banking takes place. Same thing for the connection to the server controlling my reservations (and it's not necessarily the same server). How about managing my ADRs for that DDP I got through DVC? Then there's the other stuff I might want to do on the website. And it all goes back to the user at the other end of that internet session. And that's for one user...now add 100,000 members, or 200,000. Pretty soon, we've had to increase the size of everything...circuits, CPE, servers, the number of servers, and so on.
Yes, there is a point when the basics are in place, but equipment will always need upgrading or replacement, circuits will need to be increase is either size or number, utilization techniques will need to be updated (like using WAN Accelerators), et cetera. The complexity will increase, meaning surveillance and maintenance will increase, and so will those costs.
I know this is a long winded answer to your question, but unfortunately the answer isn't a simple black-and-white one. Sorry to bore everyone, and sorrier still that I can't provide all the correct answers. This stuff is complex, and I only know a part of the overall picture. Hey, I only sell this stuff to the people who really know their stuff. But since I sell this stuff, I have a good idea of the costs.
I wish everyone a good day.