I'm confused by your repeated comments digging at the RMT here - the guarantee of a guard on every train was agreed as part of negotiations between the company and ASLEF over a year ago. This was based on what drivers wanted (not having to deal with out-of-course situations alone, etc.)I agree with your first point, but the RMT are not the answer nor the arbiter and the reality is that safety is the responsibility of other bodies of which more than one ruled on this- this has been covered in threads passim, but to summarise, the decisions on DCO safety and the various idiosyncasies and exceptions have all been made and the rulings are in the public domain. It is not relevant to start dragging in "passenger safety" yet again. I accept your point about traincrew availability, but your remedy of factoring in redundant idle capacity of surplus staff all over the place sat around on the off chance of a colleague going sick is impractical and commercially crazy, when the actual train does not even require the staff member in order to run, and it is not a fault of "planning". Does every bin lorry have a guy in the back reading the paper in case the driver sprains his wrist? DOes an operating theatre have a spare doctor in the canteen in case doc A gets a tummy bug? It doesn't happen anywhere else outside the railways for a good reason. Every single thing in life has a propensity to fail, from your doorbell to your car to your TV, so until you equip yourself with two of absolutely everything you ever need from cradle to grave it is not a good justification for carrying bodies round the network on every single train "in case something fails". This is all dragging back old arguments that have been done to death in other threads. This is not about "passenger safety".
Also - if you stand to lose a lot by not having redundant capacity, then yes - it makes perfect sense to have redundant capacity. The cost of a cancelled train is orders of magnitude greater than the cost of spare staff.