Hartley's always been pretty bad for Northampton, yet he seems to do a much better job for England. As much as I think George is a better player, and Luke Cowan Dickie too, there's logic to sticking with Hartley.
Eddie Jones seems to select his bench first and George is an oustanding impact player so he'll probably want to stick with that. England have had good results since Jones took over so he's not got a hugely compelling reason to change much.