I agree that once contactless reaches Stevenage, the LNER only fares will most likely be withdrawn.
The LNER tickets really ought to be withdrawn. The saving is small and I strongly suspect that most of them get used on the wrong train.I think they could be retained as paper/eTicket returns if they undercut the Oval fares. Arguably, that isn't in the spirit of Oval but that's another matter.
The (GTR operated) TVMs at Stevenage bizarrely give more prominence to the LNER only fares than the Any Permitted fares (displayed as ‘.’), despite there being many more GTR operated services between Stevenage and London than LNER services.
People travelling between Stevenage and London and Stevenage buy a ticket and get on the first fast train. Most won’t know or care about the difference between ‘LNER only’ and ‘.’
Even if a passenger boards the correct train on their outward journey I’d bet my mortgage that on their return many end up on a GTR service instead of LNER.