Of course any network with "complex" fares requires some en-route checking: barriers by themselves are never going to provide a complete solution!
They actually provide a potentially less complete solution than manual checks at a station. A barrier can't stop someone using a railcard discounted ticket without a railcard, someone with an advance ticket getting on the wrong time train or someone using a ticket which isn't valid for the train they are boarding e.g. boarding a Virgin service with a LM only ticket. If you have a railcard but didn't have it on you would you rather have to go to the ticket office and buy a full price Off-Peak ticket after a RPI asked to see your railcard or be made to buy a full price Anytime ticket on the train after the barriers let you through or be threatened with prosecution at your destination station?
As for the evaders' changed approaches, the first when detected is a slam dunk RoRA prosecution with (possibly heavy) fine and criminal record,
And how do you ensure only the genuine fare evaders get penalised and you don't finish up prosecuting passengers who didn't realise they made a mistake and putting people off using the trains in the process?
the second demonstrates the deterrent effect of barriers and will only be adopted by the very determined who are always going to be harder to deal with.
Some people live or work close to multiple stations and even when they always buy a valid ticket they find changing the station they use is a sensible option after some 'rail expert' decides there's room to fit in y barriers (where y is a number) without considering that for the volume of passengers 3y barriers are needed even though that number can't fit in the existing station layout.
Except that barrier lines need fewer staff to supervise them releasing others to form a permanent mobile "hit squad". But that presupposes a willingness to commit staffing resources to revenue protection in the first place. The legacy of the open stations era tends to the opposite POV.
I think in the real world the cost of the installation and maintenance of the barriers is recouped by reducing the number of revenue staff.
Also note at many unstaffed stations anything more than a short peak time revenue block wouldn't happen as the revenue staff can't be based at a station without toilets for an extended period.