It seems to me the traditional act of turning up at a booking office or TVM straight before travelling is about buying a ticket.
But the app-based and web-based systems are about booking a journey.

It's not unusual to see someone buying a ticket on the train mentioning that they had tried to buy a ticket but it was 'too late'. I also see people worrying about catching their 'booked' train, when it turns out they have an Off-Peak Return or whatever; they just think because they bought 'in advance' and have an itinerary attached that it's 'one of those tickets'.

If the big idea is really to replace a significant proportion of the booking office purchases with app usage, the apps need to offer 'buy a ticket now', without messing around with the journey planner. They can pop up a 'not valid until XX' or 'check peak validity' warning when needed, just like a booking clerk can.

 

 
I recently watched one of those TV programmes following revenue protection officers (it featured Nottingham Trams and London Underground). You can see why officers get rather conditioned to not believing excuses.

However, ticket issuing systems are not perfect...

The scenario: the time is 11:32 on 12th August 2026 and you are trying to buy a ticket online for immediate travel Westbury to Exeter St Davids. It should be easy - the timetable says there's an 11:58 direct to Exeter. Let's start with the front-end gateway, National Rail:

Problem 1: the 11:58 is cancelled. Helpfully, you can click on an option to find alternatives "real time".
Wonderful: it shows the 10:48 to Torquay is running late and is expected WSB at 11:53; pretty close to the time of the cancelled train. You can make that!
Problem 2: National Rail says it is unable to find a valid fare. "Please replan your journey".

Let's try GWR instead:
Problem 3: The GWR website has a drop-down for time that only allows increments of 30 minutes. 11:30 is disallowed as in the past, so 12:00 has to do...
Problem 4: The train options given are all after 12:00 and the delayed train you are seeking isn't shown.
Problem 5: The "earlier" option does nothing.
Aha! brilliant idea... "arrive by" 15:00 is allowed, works, and shows the delayed train.
Problem 6: Trying to select the delayed service has no effect (clicking on later train options gives the fares).

Now YOU know the fares system and that you could select any later service with the right fare and just buy a ticket for that. But for the average traveller?

Maybe today the WSB ticket office was open, or the ticket machines working (I've been there when neither was available). If they were not, it would be an... interesting... experience to board the delayed train and try explaining to the revenue protection that it was not possible to buy a ticket before boarding. (For a start, it would be nigh-on impossible to replicate the problems).

[ The circumstances are referenced in https://www.firstgreatwestern.info/coffeeshop/index.php?topic=32355.0 ]