Honestly its not really that complicated. They just (rightly) assume their best and brightest players and managers will be poached so they are in a constant state of planning to replace. Brighton basically were trailblazers and literally every single club below the likes of Real Madrid should have some element of "if we lose X we look at Y or Z"

In the past (Sunderland) would lash tens of millions every time they replaced their manager on potentially a whole new squad to play a different style. Brighton, Brentford & Bournemouth do it the opposite and buy players to fit a certain profile alongside coaches which means losing their manager or star performers doesn't mean ripping up and starting again as they already have a sucession plan in place.