free of any inherent dangers...
you mean like water to breathe right?
At the end of the day, all we can do is attempt to mitigate risks. There is no possible way to remove risk from SCUBA, and as was pointed out earlier, bad things can happen even when you are close enough to touch, or even touching your diver. The discussion on reducing ratios is a difficult one due to the amount of money made conducting these classes.
For example, an open water class lasts 3 days, takes classroom space, access to a pool and travel to open water sites. One set of gear and one instructors time is taken for the duration of the class. Say approximately 30 hours are put into the program, and the class was 500.00, that is 16.00 per hour. Now, a typical DSD program is 3 to 4 hours, and around 100.00, that is 25.00 per hour or better. When you get into other expenses, the difference is even more obvious. It is hard to get a shop owner, who is looking at the bottom line to want to reduce ratios because that cuts in to profit.
In perfect conditions, with the best students you can imagine, is 4 divers unrealistic? Nope, and the training agencies, and common sense dictates that if warranted, reduce ratios. Is it the fault of the agency if this suggestion is ignored by the instructor? Sadly, no... Personally, I conducted DSDs for years, and held to the 2 to 1 ratio in open water, and conducted most of my programs in confined open spaces with assistants just to protect myself. We need to focus more on teaching Instructors how to assert their personal standards into the classes they are conducting, and not just rely on the mandates from our training agencies to do our thinking for us.