Um... The one of the least subsidised part of the UK railway system happens to be in the SouthEast, it won't cost the company/Government anything all it'll mean is the Key ITSO card will be loaded with a pre defined number of returns and then you'll have XX number of months to use them with a 10% - 15% discount when you load them
Yes, your report shows that Southeastern was subsidised to the tune of 7.6p per passenger km during the 2018-19 year. And that is with the sky-high fares the MPs describe. So, quite logically, if people start expecting the company to offer the same percentage discount as a season ticket offers for 5 day commutes, for a 2 or 3 day commute, then the subsidy will be even higher. It's not about the cost of implementing the fares themselves, that is minimal. It's about the financial implications of allowing season ticket discounts on less than 5 day a week commutes.
Of course, the above figure can be thrown out of the window with current passenger loadings being at most a fifth of normal. So the subsidy is currently at least 38p per passenger km, and unless rail succeeds in attracting back passengers it will remain at that sort of level.
A 10-15% discount on carnet returns won't cut the mustard. The discount will have to be a lot more than that. On many season ticket routes, a season ticket costs 3-4× a day return. So you would need to offer 20-40% discounts to make carnets equivalent.