They both suited the eras they played.
First and second books were mysterious, calm and mostly introductions to the magical world. Harris played that role to a tee.
3rd book is a tricky one. It has essences of both, Voldemort isnt yet properly introduced, yet the story definitely gets much darker than the previous two. Either of the Dumbledores would have worked great with this film, as they both wouldve been playing to their strengths and weaknesses in PoA
4th book onwards, the story got darker. More tension, aggravating each other and all together trying to fight for their lives. Theres Gambon's speciality. Harris would have been too fragile, and definitely close to the end if he hadn't sadly passed 2 years previous, which wouldnt have suited Dumbledore's attitude in the books from that time.
They wouldnt have suited the other way around, so I'm glad they worked it like that