Massivebus sell you a ticket from Stop a - stop B and drive between the two dropping you off safely. Their responsibility for you ends at that point
Massivetrain sell you a ticket from station A to Station B.
Sadly the line is blocked and the train had to terminate at an alternative station. It is very wet so the line is flooded. Massivetrain arrange a bus replacement service for you but you have to walk a little way to reach it before you continue on your journey as the road to the station has flooded. you get off the train and walk to the bus and enjoy a nice ling ride to station B
Only when you reach station B safely is the duty extinguished. At all times between the 2 you are still in their care. If something happens to you whilst walking to the bus or the bus crashes etc etc Massivetrain have failed, prima facie, to have adequately discharged their duty.
I think the point about arse-covering is essentially the valid one, and that is the reason why TOCs might not want to offer a service. However I do think that you're being rather dramatic about it, which is funny given the number of times I've seen you level this particular charge at others making less fuss than you have here. It's clear that there are times when buses will not be departing from anywhere near a station and people tripping over when trying to get to the bus isn't really anything to do with the train company if they're advertising that the service will go from elsewhere. I wouldn't be totally clear on the legal position, but then I don't trust you because you're obviously just enjoying making a fuss and calling other people foolish, plus haven't offered much evidence.
I think that there is a strong element of the problem being that there is still difficulty in sourcing buses. So yes on the whole you are probably more right than Neil for the moment - at least until conditions improve and more buses might be available. But you're the one (as you and several others just love to do on these boards) criticising people's intelligence over it so don't consider yourself to have won! That's all I need say on the bus-replacement saga!