Well, since you listed examples of games that did change for the better (though it is arguable that RE changed for the worse). I'll list examples of games that didn't change and maintained or increase success & quality: Halo, Mario Kart, 2D Mario, 3D Mario, GTA, CoD, etc. I'll also liist game that did change and performed worse worse: Socom 4, Dragon Age 2, Resistance, etc.
As you can see, some games change for the good. And sime change for the bad. Also, some stay the same for the good. And some stay the same for the bad. changing the core isn't that big of a factor for a game to mantain success. Changing the core isn't that important. The important thing is that the quality stays high and you allow enough time in between each installment. (Both of which the HoW has been achieving so far). If you're seeing bigger and bigger success critically and financially), that is not the time to change.
You don't just change for the sake of changing. You change when your current methods being to stagnate or fail. This is currently not the case for God of War. When God of War does start stagnating or failing, that is when you change. However, as of now, fans and critics love GoW's current core, so it would be wise to keep the core the same.
again not set n stone true. Why do ppl complain about CoD, assassins, Pokemon, Madden,Mario ext? These IP's all sell very well but remain general the same. Just because an IP is high seller does not mean players do not expect change, and having a high seller should not be an excuse for a developer to rinse and repeat. As a developer you should be driven to renew the way gamers play regardless of Sales. If they dont then you get Dynasty Warriors.