Yes, equipment & trips is part of it. Divers need these to keep diving and to enjoy the sport. As far as pressure sales, well, I know there are shops that do that. I don't happen to run a shop like that. I've been known to talk someone out of a purchase I didn't think was right for them. I might be biased a little, but I carry what I think is the best equipment.
I run a small shop, but we have 4 instructors. We pride ourselves on personal service, working around peoples busy schedules. I think a lot of shops have forgotten that we are here to serve the public, not the public serving us. We do whatever we can to make customers happy. This month, we had several different classes running at the same time, something an independent would have trouble with. We have enough gear and manpower to teach a dozen divers at once, something I couldn't do as an independent. As an independent, you probably work a full time job outside diving. So if someone can only do classes in the middle of the week, an independent is going to find it tough to meet their needs.
Yes, air fills, nitrox fills, equipment servicing are also part of this deal and something that is difficult for the average independent. So, I do believe its a disadvantage being an independent. I didn't say it was bad being an independent, just tougher. I had a working relationship with other shops, that really helped me. Rob, I'm sure you're a fine instructor and you take folks on trips giving them experiences. But even you have to send 'em to a shop eventually to meet all their needs.