I travel regularly from Lewes to London. Super off peak, with a railcard a day return costs £15.95 and the trains are usually busy and often very busy mid to late morning.
Whenever I travel in the ‘rush’ hour, where a day return can cost £50 or more, the train, like my 06.18 today, is mostly empty. Most bays of four had just one person in them today.
Similarly on the ‘peak’ 08.33 from London to Manchester I’m sitting on. My one way ticket, using a split at Stafford, has cost £105. Yet the train is almost empty.
Why aren’t train companies or the DfT doing more to incentivise travel on services so lightly loaded?
The DfT and some people at TOCs are very keen to extract as much money as possible from people who take these early trains which are less busy, because those who use those trains often have no choice, so are there to be fleeced.
The rail network in this country isn't run on the basis of social benefits, modal shift, etc but on maximising the amount that users of the service have to pay.
Furthermore, the DfT are very keen for the cost of Off Peak travel to increase, rather than the cost of Anytime travel to reduce; you see this on the East Coast Main Line, where LNER's price trial has involved abolishing the lower priced fares, forcing people to pay more, with the only alternative being Advance fares.
TOCs like people to use Advance fares because then they can withdraw them on a whim, reduce the quota of them, and of course anyone who misses their train (which may depart 2 minutes early) has to pay again, which is a nice little earner.
Morally, your argument is quite right, but the way the network is run in reality is at complete odds with what you propose, sadly.