That only makes sense if it's based on the arrival time, not the departure time as you suggest.
A long distance train could potentially make up a 15 min delay over the course of its journey and arrive on time.
Meanwhile a 5 min delay to a single train could result in a delay of an hour or more if a connection is missed.
I fully agree though that there's a need for a solution here. Entirely predictably there is now a thread about someone who, faced with a delay of over 2 hours because they missed a connection and two different TOCs refused to help them, ended up purchasing a new ticket and will now have to argue about claiming this back. An unacceptable state of affairs that will only put people off rail travel.
Unless somebody invests workable crystal balls that doesn’t work.
15 minutes at the departure point is the sensible cut off. It is rare a train makes up that sort of delay and more often you start to miss connections/get caught behind other trains etc and so the delay increases. That sort of delay is also where the railway thinks it’s okay to terminate short of destination or skip stops and so I absolutely should be able to take immediate defensive action to prevent an even bigger headache. I usually find anything over 10 minutes seems to trip into difficult to recover territory. Also, it is not about this being an exact science. It is about saying, here is a point in time when a passenger is entitled to have lost confidence that their booked train is going to perform is function of getting them somewhere for a particular time, and so they can use alternatives.
There might be times when you say, actually I will wait for that one. If you prefer delay repay because you are not in a rush. If things are generally up the spout etc and so you think at least I might get a seat on that one when it turns up. However, I cannot bare being late for meetings and I really do think it is reasonable to be able to make a decision at that point.
To be fair, most guards get this anyway. There has been the occasional guard who has tried to lecture when I have caught another train and I have quite quickly lectured back that I couldn’t care less which company is involved, the railway has made me late despite me handing over a small fortune for the pleasure and I appreciate their efforts in helping me undo that as much as possible in the circumstances.