Might it not also increase loadings on the already-busiest peak hour trains? After all, the whole point of off-peak fares is to encourage people to travel at less busy times!
It might, but I imagine that will be one aspect of the trial, to see to what extent that does happen. I'd expect the effect to be limited, as I think most people travel at the time that suits them, and pay whichever fare applies. I can't imagine much of the leisure day out market leaving home at 07:00 just because the fare is now the same as at 10:00, though they might (say) travel at 09:15 rather than delay until 09:45.
No doubt if peak crowding gets worse, Scotrail will take account of that at the end of the trial.
It would be interesting to know what criteria Scotrail have set for the trial to be considered successful. Obviously if journeys and revenue increase, and by more than any cost increase, that will be a success. But if say ridership goes up but revenue down? Or indeed vice versa? I can imagine that a return to the status quo ante at the end of the trial might be quite unpopular once people have become used to the one price system.