There's almost no chance James would agree to an extension given that he's a FA with zero strings attached come 2021. Sox can only buy out one arb year and he's not going to give much of a long term discount for that. Would 4 years 50 million get it done? Probably, but that's way too much to commit to a guy that until now looked like nothing more than a decent backup.
Better to pay him 8 million or whatever he'll get in arb next year and continue to get more data on if his true talent level has changed enough to make a long term deal sensible.