I remember before SDO was in place, but in some ways it was easier, as there was no requirement to open and close from specific points on the train, so it was quicker and easier.
Doing a full check on a HST takes a long time - most of my services have a maximum of 25 minutes between stations. On a full train, I might get two full carriages done in that time. Less if any issues like "can't pay won't pay".
If the company was that concerned, then HST's would see some revenue staff. I still don't need both hands to count the number of trains I have worked that revenue staff turned up on, since the beginning of this franchise. Every train they have been on they have found business. And I have NEVER seen revenue staff on-board east of Reading.
I've travelled between Paddington and Cardiff quite a few times in the last few months and will be again in the next few weeks. On many of those occasions, there was no ticket check between Paddington and Newport, if at all. Once I did get on a train from Reading to Cardiff wearing a pink/purple/blue tie. The TM said I could sit in 1st. I changed by clothes during the course of the journey and when the TM next saw me on the approach to Cardiff, accused me of impersonating a member of staff. To which I just smiled and got off the train once the doors were released. Other than that though, ticket checks did seem to be quite thin on the ground.
Another day when I decided to visit Marlow, my ticket was checked on the train to Bourne End, again on the separate service to Marlow, back on the train to Maidenhead then the same member of staff boarded the Reading train and checked my ticket again. I think it's quite hit and miss.
At all TOCs I've worked at, guards on trains were never obliged to check tickets or get into conflict with fare evaders. I wonder if it is the case that any guard at any TOC is and for those who it isn't, why bother with the stress of it. The time spent arguing with a fare evader could be spent assisting a fare paying passenger in genuine need of assistance. As for delaying trains, I don't think that situation should ever be created over a fare dispute, whether it be simply because the fare evader refused to pay, or because of the continued challenging of a fare evader leading to a situation where it's necessary to get the police involved. I should think the job has enough responsibility without the stress of dealing with all that.
At the end of the day, safety is paramount and it's in nobody's interest to delay trains, so I don't understand why there are people in the grade that are prepared to compromise both, unless their personality is incompatible with their job and they have some incentive to make sales.