I think Michael and Tom would eventually have had a falling-out - Michael is clearly testing Hagen's loyalty in the second half of GF II, and Hagen, in some subtle ways, shows some dissatisfaction with this treatment.

IMO, this situation would have come to a head a few years after GF II, or the mid-1960s more specifically.

Pacino and Duvall are now way too old to play Michael Corleone and Tom Hagen in the 1960s, so I don't think it's practical to include these passages in the saga. I'm OK with recasting parts, but not in the "middle" of a character's lifespan. I don't want Robert Downey Jr. playing Michael Corleone or Anthony Edwards playing Tom Hagen in 1966...