Stargate went on WAY too long for several reasons but the one you brought up is my main problem.
As the show went on the team came up with some simple weapon to kill everything, either the Kull soldiers, gou'ld, Ori. It just felt really uncreative. It's like 'What's the point of watching the journey when the ending will feel forced, rushed and unnatural".
And speaking of underused characters, what about the invisible Kotu? Talked about them a few times, saw their invasion force and nothing came from it.