If a group of passengers constantly got on a bus service, got told their fare was £5 and paid £1.50 and sat down, no right minded company would change their service to suit them, if it was to the detriment of those that got on and paid the correct amount. That's essentially what's happening here, albeit with someone else picking up the tab.
Given the poor reimbursement rate, Trent Barton might nit be trying anything in the basis that they actually benefit. If we generously say they get 50% of the single fare, in this case £2.50, they only need just over half the amount of OAPs to earn the same amount. Even fewer if the reimbursement rate is lower, quite likely judging by the conversation so far.
Evening peak journeys are often approaching full at present with a mix of paying customers and over 60s. The new system means they either not on there or paying the full whack (the £4 ticket not being available during evening peak) which saves room and prevents frustration from those who actually keep the service financially sound.
And even better, they can blame the nasty authority for it all so there's no backlash. Why go to the effort of slowing down Red Arrow or dedicating assets to different services for the sake of people who don't pay themselves and those that pay on their behalf don't cough up what they should?