I agree. However I think it's the most likely outcome.
The ticketing system now is a mess anyway and I think that eventually all of the through National Rail - Underground tickets, cross-London tickets, and interavailability will be abolished (except where Underground / National Rail share the same tracks where it's impractical to do so)
Especially on the cross-London issue, there are a lot of Maltese Cross tickets issued to journeys where the London crossing is unlikely to be made on the Underground, just because there exists a variety of permitted routes from different London terminals for a cross-London journey, which means a lot of revenue is lost from the TOCs to TfL as well.
For example,
- from Kilburn High Road to Lancaster, the normal route is to get in and out of Euston, no Underground needed, but because there is also a permitted route out of St Pancras the ticket comes with a +.
- from Cricklewood to Brighton, the normal route is to use Thameslink to cross London, but because the ticket is any permitted and Cricklewood - St Pancras - Victoria - Brighton is also a permitted route, the ticket comes with a +.
- from Cricklewood to Kettering, with a ticket routed via London, the only permitted route is out and in St Pancras which means there is no sensible journey opportunities involving the Underground, but the ticket also comes with a meaningless +, leading to meaningless revenue lost to TfL.