To me the basic principle of a discount of x for a ticket allowing n journeys within d days is sound. What is critical is ensuring that 99% of journeys are checked / counted by either barriers, platform staff or on train staff.
The problem is applying it to a system that needs totally reforming to be much more consistent.
Take Winchester - Waterloo as a bad example. The distance is around 68 miles.
The anytime open return is £80.40 and the day return £76.80.
The off peak return is £40.90 and super off peak day return is £37.20
Then a super off peak weekend day return at £34.80.
The former is an expensive ~60p/mile while even the latter is ~25p/mile.
The annual season ticket is £5580.
For 200 days that is £27.90 per day - yes £6.90 less than even the super off peak weekend day return.
Cut the 200 days to 100 days then £55.80 per day.
8 days @ £76.80 in 28 with even 20% discount is £61.44 per day ..........
The core issue that all the one journey tickets need to be if not 50% 40% cheaper.
Using 40% 8 days in 28 with 20% discount would then be £36.87 per day, which is ~33% more than the £27.90 per day for 200 days on an annual season ticket.
Yes, you aren't limited to getting one per 28 days.
Conversely, if you end up not using your 8 tickets within the 28 days (or indeed even before the 28 days are up) you can get a refund. You'll be refunded the difference between the cost of the "Flexi Season" and the relevant number of Anytime (Day) Returns, less a £10 admin fee.
With the admin fee once 6 of the 8 journeys are used for the some refund will be zilch or so small not to be worth the hassle.