it wasn't just the final series of GOT that was bad though, it was a steady descent at the point they started writing past where the books had got to. Characters like Varys and Littlefinger suddenly started making poor stupid decisions whereas earlier they had been so clever in all they'd done.
I think it's just the final series is so absolutely jarring in it's disrespect for the viewers it's easy to latch on to that being the point it all went bad when in fact, it had been declining in quality since around season 5.