Fisher died after 8 was filmed. So her death was not what messed up that movie. However, it may have messed up 9 quite badly since the rumor was she was supposed to be featured heavily in it.
8 was bad because the script was a mess and poorly executed. And Kathleen Kennedy feels the need to jam SJW stuff constantly down your throat.
My opinion, I think they made a mistake with these movies. I personally liked Rogue One and Solo quite a lot....Far more than these new trilogy movies. Disney could have done far better by doing spinoffs and side stories like those two and just let the Skywalker saga die at Episode 6. George Lucas had already left a bad taste in everyone’s mouth after the prequels.
I think Disney kinda had to do 7,8, and 9 after they bought them from GL.
No one would of bought tickets to see Rogue One and Solo if 7,8 and 9 were also going to be made.
What was wrong with 8 was Rian Johnson.
TPM sucked! AOTC and ROTS actually felt like SW.