Other than the M and then L outages, I don't think there is any need to kill J/Z express service just so a relative few can transfer to the G. The J/Z is enough of a schlep as it is.
I do think at least some M's should be extended to 179th, though once the R179's and some R211's get here, and the R42's and R32's go away, there will not be enough 480'-capable train sets to cover it, unless the C gets full length trains, which is not needed either.