OK, another scenario based on observations between real stations.
The single TVM at Grindleford is down a long ramp to the Manchester bound platform. Most journeys originating there are down an equally long ramp to the Sheffield bound platform. Consequently not a few find themselves on the platform with no machine.
Train arrives and passenger boards ticketless. A dispute starts about the issue of tickets. The train arrives at Dore & Totley. The doors don't open. The signal is green to clear the single line section. The guard stands his ground. The dispute continues. After 2 or 3 minutes a truce is declared and doors are opened.
There's only one platform at D&T and one TVM which is heavily used and frequently out of order. Maybe 15 more join the train.
Guard returns to battle and finally sells ticket to Grindleford passenger and checks 5 of the D&T boarders in rear carriage. None who boarded at the last two stations in front carriage are checked. Train arrives late in Sheffield platform 2c with 4 minutes to turn round. Sheffield is not gated.
Guard and driver swap ends and depart on time.
Guard could have finished dealing with awkward passenger before releasing doors at D&T. That would have blocked the single line section for a following TPE service or an EMT in the opposite direction. It would also have lost a path onto the mainline into Sheffield, or delayed another northbound EMT or XC train there.
If the guard held the doors closed on arrival at Sheffield until the whole train was checked he'd have a lot of very unhappy people. The train would then miss its outbound path due to delays, and the knock on effects could be considerable.
Calling BTP might be a possibility, but at what point? How long would it take to explain? And how quickly could they drop everything for a fare evader?
In practice it's very common for tickets not to be fully checked on this section. Some get promise to pay tickets and don't go to the Sheffield booking office if they've avoided the guard.
Of course Northern could cordon off 2c at Sheffield, an East Midlands station, but that's not normally done. Holding the train anywhere en route would add to existing problems. Punctuality is so bad on the Hope Valley line that all stations are currently in the worst 100 in the country for that.
There are lots of ways to maximise revenue without blocking tracks. Ticket machines that work, are positioned where they're most needed, and are simpler to use. Encouraging more to use apps, season tickets, and online purchases will help.
On trains conspicuous spot checks by RPIs backing up guards may get the message through. Unfortunately for every one confused irregular traveller there will, sadly, be at least 5 more who are trying it on!