It isn't simply lower demand, but also higher costs due to the nature of the multitudes of ageing diesel rolling stock operated. Investment in modern rolling stock can reduce costs over the longer term.
There are some areas that have high demand, and these areas having the benefits of lower operating costs could just as easily see returns, particularly where the modern rolling stock and better services encourages patronage.
£1,844 for a Zones 1-4 Travelcard is exceptional value and I would argue easily "out values" your comparable tickets in other PTE areas.
A diesel train has an "other costs" (Energy Cost, Track Maintenance Cost, Electrification Fixed Equipment Maintenance Cost) cost per vehicle mile of 54p, versus 33p for electric. Add up the amount of diesel trains operated in the north and the sheer amount of miles they have to go to do their job versus the tight networks in the south, and not only can you see it's not something that shouldn't be compared like for like but also the financial benefits that mass electrification in key areas and modernisation could bring over the long term.
It's much too simplistic to say "newer stock means higher fares". In reality, it should be planned properly so that it means lower fares, more service for less money.