Inertia is the resistance of any physical object to any change in its motion (including a change in direction)
I realized something that seemed quite profound this evening. A company works on a set of scales, and one that relates in a loose (and very rarely mathematical) way to notice periods of the people hired.
A CEO/MD of a company has a long notice period, but their departure rarely has a meaningful impact in the short term, (ignoring moral for a moment of course). They seldom make decisions that effect the day to day work that is going on, or, if they do, it is something that can be covered by the teams under them. If they are out of the office for a day or a week unexpectedly, very little in the way the company operates will change because they are tasked, truly employed to focus on the strategic goals of the company. Making the decisions and commitments that shape the long term vision and direction. They have a duty to the 3 month to 1 year commitments of the corporation.
A junior, someone perhaps on a week or month notice period's sudden departure can have a profound impact on the work and commitments the company has directly committed to because they are focused on the day to day production of deliverables, and it is a rare situation where there are spare people sat around to cover with nothing better to do with their time.
The people who most impact a company should they lose are those in the sweet spot between the juniors and the CEO's and the exact spot is dependent both on the company and the team they work in, but it is a mix of the skills gap between them and their team, and the recognition the company has for their particular assets.
A person can leave who is not a core member of any management team, but who is, with their relationship both to the companies commitments to deliver and their ability to influence the strategy or one or more clients a pillar of creation, Someone who's departure is more serious than a CEO, or a bowling teams worth of juniors going their merry way, and without recognizing this corporate inertia, this assumption that you can measure people by title alone, it becomes very easy to confuse the departure of someone quite unique in that company with the departure of someone who sits on a far point of the scales.