Then the people who are 24/54/114 minutes late will have the same argument. A line has to be drawn somewhere and there will always be some people who fall the wrong side of it.
Using exactly 30 etc is unfair to more people, because if a train on a 30 minute frequency service is cancelled, you ARE 30 minutes late precisely by definition. Recovery time doesn't make that actually 29 minutes. It's grossly petty and pedantic, and acts against the railway far more than not having DR at all would.
For a non-cancellation delay yes, there'll always be someone disappointed, but most trains in the UK run on a clockface timetable and so it trips up people whose train was cancelled almost every single time.
I would suspect this is a feature and not a bug...
It might save a few quid but like a lot of things just makes the railway look petty and penny pinching.
I'd accept removal of the 15 minute tier if they fixed this, either by changing it to 25/55 etc or by processing cancellations differently so a cancelled train or missed connection on a 30 minute frequency with the following one on time is automatically classed as a 30 minute delay (etc) even if it's technically 29 minutes due to terminus recovery time (which itself is in many cases a wilful and dishonest obfuscation, e.g. at Wrexham Central where on a single platform the inbound train is scheduled to arrive after the same train is scheduled to depart again!)
To be honest it would fix it if the time used for DR calculation for any given train was the later of the timetabled arrival time or the actual arrival time, also taking into account that the recorded time is often a good 20 seconds before the doors are open at the platform (which is further sneakiness).