If someone holding a Z1-4 travelcard is regularly travelling outside of their zones then they've bought the wrong travelcard product. For occasional journeys they can decide to stump up the £1.20 or they can always take the bus.
The only day travelcard products are Z1-2, Z1-4, Z1-6 and Z1-9, and there's no off-peak version of the first two, and the price caps match these products. The fact there isn't a Z5-6 day travelcard or Oyster cap doesn't mean you're paying twice for a journey. You're paying £1.20 for each tube or rail journey you take outside of your zones.
Even if the Z2-6 Oyster cap was brought back you'd still be "paying twice" for zones 2, 3 and 4.
Single fares become meaningless in the context of caps.
You are also missing the fundamental point that paying for Zones 2, 3 or 4 again is not usually a significant issue, as opposed to the hefty premium of having to pay twice for Zone 1 travel.
What other reasons are there for the removal of the cheaper cap outside Zone 1, other than stealth fare rise? Simplifying the product range is a complete red herring as with the introduction of electronic ticketing, this becomes easier to do than ever before, not more difficult.
The simple matter of fact is that TfL over the last few years have introduced stealth fare increases left, right and centre, while shamefully attempted to dress them up as something they are not or play down these increases.
I understand why it's frustrating, but you know the limitations of the system when you choose which Travelcard to buy.
That is a poor attitude to adopt. I am sure you were not very happy when the same argument was trotted out in the context of National Rail disputes in the past.
Just because the passenger is aware of the limitations of the product purchased does not mean there is no room for improvement.
