Griezmann has played second striker 28 times this season, centre forward 20 times this season, and right wing 4 times this season. I get your point that he'd be given freedom to roam, but he's most dangerous in central areas, so it seems wasteful to spend so much on him (or James) only to shift either of them out to the wing. Plus, I haven't yet given up on Martial or Mkhi, as some seem to have. I think Mkhi is, at his best, capable of brilliance on the right (as he's shown in patches this season), and I'd like to see Martial starting on the left next season.
Bottom line, the difference between Mkhi and Griezmann on the right wing is negligible, whereas Griezmann thrives where James does - down the middle, meaning either James or Griez would, most likely, put Pog in a 2.