It's a business. If there were no margins, there would be no diving, no gear, no nothing. If you think your LDS is ripping you off, ask McDonalds how much margin they are making on the average Big Mac. A whole cow gets sold for a matter of a few bucks and then it's diced and sliced into a thousand barely palatable burgers where 10 Big Macs costs more than a whole cow. How many potatoes can you buy for the same price as a packet of regular fries?
A beef steer costs well over a thousand dollars, not "a few dollars". So beef, from the supplier is something like a dollar a pound.
For the rest of your post, I'd certainly agree that the vast majority of LDS owners are not trying to screw over customers and are not getting rich doing it. They are just trying to make ends meet. The problem is that many of them are just really poor at it.
Up until this summer I had two shops close by to me. Frankly, I didn't really like either of them.
Shop A: Barely acknowledged that you walked in the door. When I picked up some weights he figured I was into go carts because they buy weights. Didn't fill nitrox. Only gear for sale was stuff for vacation divers even though I live 40 miles from some of the finest wreck diving in the world. I asked about getting on an e-mail list for trips, etc. and he took my e-mail address. The first e-mail I ever got from him was literally "Prove to me why I should stay in this business, here's what I sell." The second e-mail I got from him was "We're going out of business." Big shock!!!
Shop B: Kind of a weird attitude. Very neatly kept with rows of very expensive gear sold at MSRP or above. They do sell some gear more appropriate to the area. They do fill Nitrox and they arrange trips out on the lake. So they get some of my business.
Shop C is one of the more frequent posters here at SB. I'm sure you all know him. The shop is too far away to buy incidentals and fills from, but I do go there for major gear. It is a WELCOMING place.
Finally, I never got this "airfill" thing. If people are diving somewhere, there will be air. The quarry where I dive has air and nitrox. It's a bit pricier than a diveshop, but probably because it's not subsidized. Some of our boat captains can supply filled tanks. If all the LDS went out of business, the rest would supply air or THEY will go out of business. The bottom line is, people will dive and where they do, there will be support for them. This may suck for those of you who dive in mud holes no one else does, but for 90%+ of divers, it doesn't matter.
So I'm happy to see half or two thirds of the shops fail if it means that the remaining shops are able to provide good service to their customers.