Where do second hand bikes
come from? Thin air? If the main dealers go under, then there will be no new bikes sold, so the stock of good used bikes falls.
Main dealerships cost a lot of money to run, but irrespective of how polished the floor is, how shiny the bikes, how clean the workshop, a dealership can sink or swim based on how it interacts with its customers.
Striking the right balance between the hard sell and ignoring potential customers can be hard for some dealers, but most get it right. The salesman has to develop a keen eye to distinguish between a tyre kicker, a joy rider and a serious prospect, but should also know how to develop a relationship with his or her customers.
I agree that a good service should not be optional, it should come as standard, so a 6000 mile service at a main dealer should cost the same, no matter which dealer you take the bike to. If a dealer wants to retain his customers, the added value to a service is cleaning the bike, offering coffee, a range of magazines to read while you wait etc. Added to that, of course, is that the bike has been serviced or repaired correctly. Fail in that, and the owner will go elsewhere. Lose the customer's trust, you lose the customer.
However, it should not be forgotten that there is a place for the back street workshop, who don't have to pay for a shiny showroom, sales staff, or warranty claims. Because their livelihood depends on repeat business, then their modus operandi is cheap labour rates, and a friendly attitude, and of course, competence. There will be a limit to what they can do, with modern bikes requiring special tools, software and diagnostics, so you may end up having to go to a main dealer anyway, but on the whole, they offer a cheap alternative for the used bike owner.