I don't see what the problem was with Harry not being the Goblin long enough.
I mean, Harvey Dent wasn't Two-Face for much of "Dark Knight", no one complained. Emil Blonsky wasn't Abomination for much of The Incredible Hulk, no one complained. Obadiah Stane was only Iron Monger for a small bit of Iron Man 1 and no one complained.
Harry was Green Goblin for all of... three minutes? He literally showed up right after Spider-Man and Gwen defeated Electro (how convinent) and then stayed around long enough to do you-know-what to Gwen. Then he was knocked out and that was it.
At least The Dark Knight
had a few scenes of Harvey Dent as Two-Face and Dent becoming Two-Face was a big part of that story and the emotional climax. Emil Blonsky transformed into The Abomination for the climax of the film, which lasted more than two or three minutes. The same with Obadiah Stane - we at least got a decent confrontation between Stane as Iron Monger and Iron Man.
Green Goblin's role in The Amazing Spider-Man 2
felt very much like Whiplash's role in the finale for Iron Man 2
- he showed up and was defeated in about a minute. Wow, that was impressive!