I don't think Morrissey was thinking of leaving BMG at all. I think he was surprised and dismayed that he was dropped - his message on Central seemed to convey that. Then he tried to blame being dropped on some kind of diversity push by BMG. He was either told that or he simply surmised it, only to have BMG sign such "diverse" acts as Louis Tomlinson and Johnny Marr.
We have seen the publicity blitz BMG put behind Johnny's new album - a Sky Arts documentary, interviews on mainstream tv, radio airplay, dozens of podcasts and interviews, album signings, several magazine cover stories and opening slots on 2 major tours. Those are the kinds of things that Morrissey probably didn't want to do and that would frustrate a label.
BMG dropping Morrissey and signing Johnny Marr was probably one of the motivating forces behind his open letter.