Yes, this was surely somewhat tied to employee turnover and poor record keeping. But I was almost certainly not one of the "biggest API users."

The thing is, regardless of the turnover or situation, 4-days (actually 1-business day Friday->Monday) is not a reasonable timeframe to threaten to shutoff someone's API key who hasn't violated any terms of service. They have the right to do it, but it doesn't make you want to work with them in the future.