You're right their current situation is not down to one player leaving there are multiple factors. I should have said their downward trajectory began after Kante left.
But Kante was very influential, I think this Telegraph article from November is pretty good...
I think winning the Premier League with Leicester was an amazing achievement. They played exciting football too.
It's not coincidental that their downward trajectory is after Kante left.
He was integral to how they played, and Ranieri hasn't been able to replace him.