Honestly the odds are they're going to get caught. There will be a very very very small minority of passengers who just travel once, and don't pay. And they may slip through. But persistent offenders are upping the odds of being noticed significantly, found out and caught. And stations and guards soon get to recognise regular troublesome faces.
The same people won't keep getting away with it. You'd only need a revenue check once a fortnight and to catch the same person twice in the month to flag up serious problems for that person. In the grand scale they will be caught if they are a regular offender.
Think of it like this. When people park in council controlled paid parking areas, like council car parks or council streets where parking is paid for when you park there, it is not 100 percent that a parking warden will come and check your car. In fact, the odds are chances are your car won't be checked if you park for an hour. Park for two hours the odds increase. There may be a warden on duty there may not be. They may have a few days off or be sick, or be in another area, or only check that car park two days a week. But you wouldn't really want to risk parking without a ticket, as the consequences of being caught are just not going to be worth it. And nobody calls for more parking wardens. I'd hazard a guess you're probably more likely to have a ticket checked at some point before during or after your rail journey than you would be having your car checked parked in a council car park in some local areas. And parking wardens have a bad name even though they're justifying the fee the huge majority of paying users are paying to park. But people perceive the risks and consequences of being caught parking in local authority bays without a ticket to be quite serious. And with railway revenue evasion it could end up criminal. Even more serious. There are some who don't care about those consequences and they take the risk, and thousands are caught and there are court prosecutions and it's going good to affect their outside lives. Those who don't care about that fine, they're welcome d to those risks. Bit But most reasonable people would worry about that and would t want that and so the order is that the vast vast majority of passengers will pay willingly and happily.
There are probably a good many who don't realise it can become a criminal matter deliberately evading your fare otherwise they'd ensure they purchased a ticket before hand absolutely every time .maybe the potential consequences should be publisized more and that would help and make fare paying passengers feel a bit more looked after and valued than some do now. Although I suppose on the rail industry it's very customer service oriented and tocs wouldn't want to frighten people who may be elderly and have trouble using a ticket machine at an unmanned station and turn them away though fear etc and other such situations. So the balance needs to be right.