If we bought Maddison for £100m, and sold MGW for £100, five year contracts, we'd be up £75m on MGW with a £20m cost for Madison for the year. Gives us £55m free!
That's a good reason, there are others. He's younger and may be better one day. He already has an understanding with Johnson and Johnson is becoming more important for them it seems. They want more than one player in every position and are rich. They want someone with more legs than Maddison who tracks back more. Etc etc.