It's been mentioned here already, but I think the acceptance of deaths in training is an interesting one because it provides a glimpse into their society's attitude towards death overall. A lot of SciFi books go down the road of a futuristic society that tries its utmost to banish death. This is similar trajectory to how our own "modern" society has gone, culminating in our insane current state where so how the only thing that matters is reducing deaths from a virus to as close to zero as possible.

Of course our current attitude to death actually is costing more lives than otherwise due to costs of these 'anti-virus' measures, which makes no sense in a rational cost/benefit analysis. Yet the reason these can be sustained, in my view, is that the drive to reduce death as much as possible becomes driven by the ultimate appeal to emotion - people dying is the worst thing ever, so you need to be on our side otherwise you want people to die and that makes you the worst person ever. This attitude is only possible in a society that has an unhealthy attitude towards accepting death as part of life. The cold, harsh reality is that we are all mortal, and we will all die. It shouldn't even need to be said, yet it seems that such reminders are needed in modern society. Death is far from the worse thing that can happen to someone, simply because it happens to everyone - so why would we make something that is part of life the "worst" thing that can happen? Not living our lives to the fullest extent is, in fact, far worse than dying itself.

Going back to the book, it seems to me that Heinlein is proposing that attempts to avoid death at all costs, which must have already been apparent in his time, are in fact a flawed approach to a society's evolution, and that a healthy successful society would revert back to a more traditional approach where death is accept as a necessary part of life, both for individuals and their society.