Phil,
I actually agree with some of what you say. My definition of "local" may be different than most. I actually equate it more to mom and pop shops than shops that are in my backyard. I've been highly involved in the rock climbing/mountaineering community for a number of years. I have no idea on actual numbers, but I'd venture a guess that there are similar numbers of people involved in scuba as in rock climbing...and due to the relative cost of equipment, there should be more money to be made in scuba. Now there may be a smaller number of climbing shops, but I will say that in the climbing community, there is a strong grass roots effort to support these small shops. I believe it's because of this "community loyalty", that keeps these shops in business. Perhaps this doesn't exist in the scuba world, at least to the same extent.
It's obviously tempting to buy online, or head over to one of the larger shops, like REI or EMS (and I shop all of these when I feel it's the best choice for me), but these climbing shops are run by climbers, and that's important to many of us bound tightly to that community. If a new climbing rope is going for $200 online, then maybe it costs slightly more at the small shop, but it's often a difference that doesn't turn loyal climbers away..perhaps 10% tops. I've noticed price differences when comparing my local dive shop to online of around 50% at times. Perhaps the scuba to climbing analogy doesn't translate well, but assuming it does on some level, the one difference between the two is that although the local climbing shops may at times charge more than online or larger stores, what lends to their survival is that they don't completely "gouge" their customers, while offering that "one of the group comradery" that many of us seek out. Perhaps the profit margins are different in scuba, but I think this is a largely contributing factor to the failure of these "local dive shops".