A fall in sales rarely happens overnight. In actual fact it would be easier to deal with if there was a sudden drop off as this would inevitably point to a specific, identifiable reason. As you know, where there is reason there is usually remedy.
No, the more dangerous fall in sales is that which happens gradually over the space of months and years, but which can nevertheless leave a previously healthy business in a precarious position. Worse still, as the decline happens slowly it is either not noticed, written off as being insignificant. The problem is that as sales are often measured against previous year only, a poor cumulative position over a number of years is less noticeable - but just as dangerous.