Wasn't the suggestion earlier that he'd been arriving in the morning before the barriers were open so only paying one maximum fare a day - combined with dodging the gates occasionally this may have been cheaper.
He'd undoubtably have been less conspicuous with a Z1-2 travelcard.
I have a sneaky feeling that he might even have got away with it if he had done that, doubly so if he had used a paper travelcard (if they're still available) as they are harder to track. Either way, it is pretty unlikely that he would have been caught had he had a travelcard, although the lack of tap-in or tap-out had it been a (hypothetical) Oyster Travelcard would be suspicious, although there are innocent explanations for such discrepancies.
Personally, I think that he might well have been picked up on suspicious activity by TfL, and it wouldn't be too hard for a tip-off to be provided to SouthEastern although the eagle-eyed barrier attendant is another explanation. I would even dare to suggest that the quick settlement was because he managed to get away with it so easily for so long, to cover over how easy it was to get away with, and how easy it would be under slightly different circumstances.
Another problem for any criminal case would be the lack of evidence apart from a confession (which may be deemed to be inadmissible by reason of self-incrimination without sufficient supporting evidence) for all but the most recent incidences - and bear in mind that any civil case is already settled.
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> How exactly is he a danger to society?
Fraud be it rail, benefit, tax, immigration or financial is real a danger. It undermines the consensus on which our society depends. If he only pays when he gets caught why should I pay. If nobody pays then we would have no trains.
If your point is that it "undermines the consensus on which our society depends" then you might as well send everyone to prison, and this coming from someone who pays his rail fares. After all, I can think of a few other things, not necessarily criminal, that could be considered to undermine the consensus of whatever 'society' is that I have done (although they would be illegal in other countries that I visited this year).
Oh, and 'danger to society' is usually taken to be in terms of violence or deliberately inducing the fear of violence.