Unless of course you reduce the price of other products which benefit a greater percentage of the population.
I don't think you can equate the two situations.
In the peak, trains in London & Southeast are already full and standing. Most people travelling during this time
have to because driving is impractical (too costly, too slow or bad parking). Reducing season ticket prices (even if revenue neutral) will only make overcrowding worse. Given there's no extra revenue, it's very hard to justify the expense of longer/more frequent trains (i.e. more rolling stock to hire or infrastructure to build).
On the other hand, off-peak rover fares encourage people to travel when trains have space that would otherwise be unfilled. Someone using a rover
does not have to travel by train, as they are not under the same pressure to buy that ticket, so could instead spend their money on some other leisure activity if they felt the train is too expensive (and hence the railway ends up with less revenue). The same applies to Network Railcard fares, only valid after the morning peak. It's also why the quieter trains have more cheap advance fares.