Here is the one contradiction in the whole “they went to hell in a hand basket after Hackett left” argument: Hackett himself claims he left because he wasn’t getting enough of his ideas through on the albums. If that’s true (as Hackett himself claims), his impact on the music of the “golden era” was, by his own admission, not as substantial as was the input from other members.
He still left his unmistakable mark. But in fact the decay was already making itself felt with W&W... One example: the unbearably sugary 'One for the Vine' or 'Your Own Special Way' (er... that's two, innit). ATTWT has its moments, but after that there were... no more 'rock' guitars.
And that period in time saw the end of the prog rock era as a whole, definitely.