Talk to me when we're back to running 12 car trains more than every 15 mins. Peak hour fares only make sense when there is so much demand that the current infrastructure simply can't handle the demand, and as such there is a need to try and push people to take the train at less busy times if possible. GO is nowhere near this point. Whilst we have hit 100% of our weekend ridership, our weekday ridership is at 45% pre-pandemic. This doesn't even begin to scratch the fact that within the next year or 2 we're going to see our single track lines begin double track service which should further improve frequencies, and we haven't hit our theoretical maximum headways on the Lakeshore + Kitchener Lines.
Let's also not ignore the fact that GO is desperately trying to bring back it's rush hour commuter crowd by offering incentives like unlimited passes:
https://globalnews.ca/news/8773488/metrolinx-presto-back-work-pass-covid/. Whilst I am generally more optimistic when it comes to transit usage in a post-COVID world than most others, let's not kid ourselves and pretend that COVID hasn't done a major impact in the rush hour market, and has naturally distributed transit passengers from rush hours to off peak times on its own. So no, there isn't a need for any off peak discount or peak hour tariff anytime soon.