Why not bore the tunnel?

When the original project for the Canada Water platforms started, I would have been surprised if a 10 year plan including expansion wasn't considered, including future costs and practicalities.

As others have pointed out, you could also run a longer train and not open all the doors, as already done elsewhere.

Thinking again, I would guess extra pressure on the Jubilee line from longer Overground trains stopping at Canada Water would be too much for the transport network to handle though...