But what about those lost pennies of revenue!
Before privatisation did "London Thameslink" group exist on the Thameslink route? Allowing London Terminals ticket to reach the other side of London will surely abstract TfL's revenue!
I think that the simplication should go another way - abolish the London Terminals and London Thameslink group and issue tickets only to specific stations, unless the stations involved are close enough not requiring further transport (e.g. King's Cross / St Pancras remain in a group, Waterloo / Waterloo East remain in a group), with any further travel requiring a Zone U1 ticket including the Thameslink core, which are interavailable on the tube.
For example, King's Cross and Liverpool Street are completely separate destinations, and tickets to Cambridge shouldn't be interavailable. There should be two different sets of tickets from each as they are on completely different routes (buy two singles if you require travelling out and back on different routes).