Except managers are "sticky", and generally aren't fired merely because they are less burdened. All jobs not just managers are in truth sticky, but in general the more status a job has, and the more high status interactions a job has the greater the difficulty in removing the position.
A lot of companies became much more productive on paper during the recession, not because they did massive changes, but because they were forced to finally cut some staff. Like a cheesy 80s movie they had the productivity inside themselves all along, but it took the need to trim budgets to expose that productivity.
Of course a lot of companies never actually really pared down things, or not enough to really boost their productivity. It just isn't simple to get that productivity to reveal itself, especially outside of an existential threat.