Fanservice is one thing. Progressive development/evolution is another.

Twilight Princess was fanservice. And for the fans, it was great. Even for people like me who didn't play OoT, it was great. But when you look at Wind Waker--which was an evolution of the '64 games--Twilight Princess didn't seem like an evolution to some people. To people who were looking for more. I don't know how different/similar Majora's Mask was compared to OoT so I can't comment on that.

Japanese RPGs don't count. They're usually about the story, gameplay is a minor feature. Some of 'the best' ones use the same gameplay over several games, and that's ok because they're Story games, not Game games.

God of War is a Game game first. The story is fine, but if you're going to make an action game you have to do things differently/better to stay ahead of the curve.

In a competetive field (this applies to everything), in order to sustain your 'ranking' you must constantly improve. The competition is always advancing, and if you stay the same without improvement you will be overtaken.

Taken by itself, GOW2 is a very fine game. There are some people who do appreciate the qualities of a game itself. I don't think anyone can lucidly play GOW and then say it's a bad game.