At some point driver less trains will be considered which will remove the cost of a driver altogether. 
Years off, if at all. The current (driver operated trains) have lasted 40 years and I would imagine that the new (again driver operated trains) would have a similar lifeline. Unless there was radical changes to UK policy regarding the operation of trains, then I can't see this changing much either.
Not totally sure why though. As pointed out by neesyoneoneone, the DLR has been driver less since 1987.
Guessing Mainlines, where there are some complex crossovers into other lines might be tricky to upgrade, but surely the Metro was, and is, ripe for it?