The lesson would appear to be: don't give top manager money (especially long-term) to someone who is not a top manager. Let him walk. He's not a top 10 manager, and paying him like one is a mistake as a consequence.
It's like overpaying for a player, but worse. No one will take him off your hands one step ahead of the headsman's axe, so if you need to make a change you're writing a huge check. If you're redlining the finances, you have to sell a bunch of high-value players to sack him. If the manager is just picking up a paycheck like high-wage bench dross, he can do a lot more damage than one bad contract, especially if his wage is twice that of your most expensive player.
Plain and simple: they made a huge mistake by not letting him walk earlier.
I always got the impression, that he saw Leicester as a stepping stone, to one of the " bigger " jobs, same as he did with Celtic.
He`s all about himself.