In my opinion this doesn't explain why the minimum fare exists, just why it's not completely invalid. Why do Coventry to Birmingham commuters not get a discount on their commuting costs at the start of their working lives?Because it provides some relief against commuting costs for young people at the start of their working lives when they will generally be earning less than later in their career.
Take Stevenage to London as an example:
Anytime Day Return £28.80
7 Day Season ticket £122.70
A 16-25 or 26-30 Railcard holder can travel for £19.15 per day with no need to part with a large sum of money up front for a season ticket.
This is an important feature of railcards for younger people that is often overlooked.