Paul G wrote:

You're also ignoring that most London National Rail stations don't have
ticket barriers (although this could be dealt with by your point below)
and it's not viable; nor are ticket barriers closed at unmanned
stations; nor can users touch out if they didn't touch in (e.g. zone
extensions from stations outside of the Oyster area); I'm sure there's
more

You wouldn't need barriers, just machines in which Oyster users could
touch in-out. That's what they have on DLR. And that line also has
reasonably frequent inspections on the trains.

Again this wouldn't cover all situations (see above), unless there was a
flat fare style system. I would have also thought there are more
carriages than stations, so it would have extra cost implications.

I agree, touch in/out on trains is not appropriate. A texting system
would work well as an option.

Those with season tickets though should not have to go out of their way
to send a text every time they use the service.