Some points here.

Hopkins agreed to fight Calzaghe in front of a bunch of people including the then head of Showtime then changed his mind the next day.

Calzaghe fought a dozen times in England against English opposition before he fought for a title. No comfort zone there.

Rather than Calzaghe-Hopkins affirming Joe as a legitimate champion, the fight will actually show a huge gulf between Calzaghe and Hopkins who will lose badly. Afterwards Calzaghe haters will say Hopkins was 43, Calzaghe wouldn't have beaten him in his prime etc. etc.