That's a fair point, but the current system is integrating with today's infrastructure. If they want people to tap in to go London to Inverness, they'll have to reconsider how to do it.It could enable contactless to be rolled out to a wider area. If you enable contactless for journeys costing £50 one way, for example, there would be potential for huge revenue loss if inspections are infrequent if you only charge a maximum fare after an inspection. So TOCs won't allow contactless for such journeys as it is too risky for them. With the Dutch system, it is no more risky for the TOC compared to passengers using traditional tickets or e-tickets.
I don't know why it takes a long time to transmit the gateline data, but it does - whether over a wired connection or a mobile data one (some gates had TfL using an LTE router to send data until the systems could be linked).
In theory, both could be real-time but my guess is they send packets of data periodically.
Never say never, but there would clearly be a big cost. I'm sure Cubic would gladly take the money, but who pays? (Cubic is presumably happy right now with all the barcode readers being installed).
The current system works okay IMO.