What was the discount on the carnet from Luton compared to an anytime day return? I do know the flexi seasons from south WCML stations are/were spectacularly poor value. It might well be that it's possible to beat the per-ticket price on the carnet, possibly substantially, by buying a single in and a Network Railcard discounted single back, enabled by the single fare pricing. (On some routes, though I don't know about this one, this was made uneconomic by singles not being half the returns).
Early starters will also appreciate being able to travel in off peak if they touch in before 0630 (in practice I think there are a couple of minutes leeway on this) which isn't *that* early from as far out as Luton. Free breakfast, effectively?
It's swings and roundabouts. Some pay more, some pay less (some substantially less!) - on the south WCML I'm almost certain I read somewhere (though I forget where) that it has been slightly revenue negative, so certainly no profiteering!
The one I do see as an issue on GTR routes is the weekend super off peak which has caused a big price increase for leisure travellers. I'm not sure how you fix that bar not rolling out Oval, and it's not something that applied to the WCML in the same way because our super off peaks were very heavily restricted so of far less utility.