Delay repay was to some degree a political sop to the constant complaints and angry letters to MPs that train companies were being compensated by NR for delays due to track signalling, etc, but none of that was being passed-on to customers.
But the 'knee-jerk' way that delay repay was implemented went beyond addressing that particular perceived unfairness. It appears to give customers an expectation of compensation for arrivals later than those advertised at the time the purchase was made. That includes arrivals which are later due to timetabling changes for which the train company doesn't receive any compensation from NR.
If compensation is payable when delays are due to timetabling changes rather than failing to perform according to the timetable, it creates the perverse situation where there is a disincentive for the industry to introduce timetabling changes, even when those changes would benefit customers as a whole. e.g by getting infrastructure maintenance and route improvements done in a more timely/cost effective manner or recasting a service to provide better connections, journey time reductions etc.
I don’t care why my train is late - I have purchased a ticket to travel based on the timetable that is offered. If that train is delayed, I have a contract with that operator, and it is for them to address the underlying cause, whether it is directly within their control, or due to the failure of a subcontractor to deliver their bit of the service.
At the moment, there are circumstances where that’s a bit hard on the TOCs. At other times, it may work in their favour. As a customer, I regard Delay Repay as a broadly fair approach that acknowledges the experience of customers in a no fault way. It saves them from receiving and having to pay staff to process stroppy letters, many of which will now cite (rightly or wrongly) the Consumer Rights Act, and leaves them space to deal with the genuine exceptions that need special treatment.
From what I see on here, it seems that some are better than others at doing this; my experience has broadly been positive and even the one tricky case (split journey involving XC) was paid quickly and painlessly - though I did take care to make sure my scanned ticket also had a letter setting out, simply, my planned and actual journey.