Great players change the game.
I grew up in the late 80s and 90s thinking Sampadoria were one of the giants of European football.
They weren't. They were Italy's equivalent of Leicester City (edit: they were better than Leicester, and for longer, but they came from nowhere and went back to nowhere).
They've won 6 major trophies in their history, all of which were in that 80s/early 90s window.
It started as soon as Vialli arrived at the club, and the final trophy was just after he left. Few players have that sort of impact or legacy, and he was the figurehead of that golden age.