It is true that people can get their feet wet and then find themselves immersed a bit more deeply - locally involved people gain responsibility from year to year, for example - but the progression from local organiser to "stable conference staff" isn't always as easy.
Firstly, local organisers can get burned out and feel that once someone else is hosting a conference, they're happy to take a back seat. Secondly, if other local organisers are happy to do a lot of the "stable" work, then it's difficult to see why one would have a "stable conference staff" until the venue has to move again. I still have involvement in a conference where, like linux.conf.au, there are challenges in finding another host in a couple of years.
As for conference size, it isn't necessarily easy to add rooms. Conferences have to be pretty careful choosing the right size venue unless money is no object, and in certain situations there's no possibility of adding rooms without moving up to a much bigger and different kind of venue. Once you get up to the thousand person conference, there are serious financial risks involved.
Some conferences have addressed the issue of becoming too big by having a cap on the number of attendees. You can also raise prices, too, but that risks turning community conferences into corporate affairs with all that this brings with it. And yes, I would imagine that most conferences of the nature of linux.conf.au discourage "at the door" registration with much more expensive pricing and, indeed, the risk of not being able to get in at all.
If you're really interested in going to a particular conference, you sign up while there are still tickets. If you miss your chance, there's a growing consensus that supporting another conference on the same topic is not only the next best thing, but also the right thing to do.