Barriers work more than you think. Evening trains especially, people know the barriers will be open and get on without a ticket based on knowing that their destination the barriers will be open. For a good number of years, back when tickets were sold on Merseyrail, it was common for people to get on along the Wirral line and by the time the guard got to them, they would state Spital as their origin. On the return leg, they knew the barriers would be open so wouldn't get caught (I know a number of people who did this).
Rail Replacement buses too, as drivers aren't allowed to check tickets and coordinators rarely do, people freeload on them. As soon as there is a 'barrier', in this example, a person asking for tickets, suddenly a lot of phones come out and a lot of tickets which are bought to the station (For arguments sake, people get a ticket from A-B on the train, if challenged, knowing that B-C is a bus. All too often, when challenged, people get on replacement buses saying 'I don't need a ticket for the bus' or 'it's a free service' etc etc). As soon as tickets get checked, it's amazing to see the amount of tickets checked and amount of people who start walking (if stations are close enough).
As soon as people know tickets won't be checked, they freeload. Barriers are a big deterrent. As has been said about Abbey Wood, now there are barriers, people are walking further to get to a station without barriers so that they can fare evade. That is as good evidence as you're going to get that barriers stop evasion.
In my opinion, we need barriers open for longer hours and TOCs need to take a bigger stand against fare evasion, not having the railway completely open for free travel pre 7am and post 7pm (when the majority of barriers close/open respective)
We need for the hundreds of additional agency staff, which TOCs now like to employ, to start enforcing tickets. I know that some operators have travelling security on lines which can have trouble. Why can't these start enforcing tickets as well. If a TOC has a guard travelling as a passenger, why not put them on revenue protection duties in the front 2 carriages while the normal guard controls the doors and enforces the rear carriages. Make use of the staff that are already being paid to be there.
Until TOCs start taking fare evasion seriously, fare evasion will keep happening. Same as trespass and vandalism, people get away with it or a slap on the wrist and so the message sent to society is that it's ok to do these things.