I'm just not sure we would have the same Willow in s8 if Tara hadn't died. I mean, maybe she would've still gone dark or flirted with dark forces (like she did in early s8), but I don't know for sure if she would've gone over the edge like she did at the end of s6. It's also very possible that if Tara had survived, she would've become a more powerful witch. I think she had a lot of potential there, but the writers didn't develop it to further contrast with Willow's obsession with power. If she had lived, she might've been able to help Willow to rein it in, and Willow might've helped her develop further. So I can see her in the thick of the battle with Willow in season 8 and see her & Willow coping together with the loss of magic. Kennedy's attraction to Willow's power was not good for Willow.