This is exactly what happens. So then why not just include it in the salary at that point? All you are doing with bonuses is just causing an opportunity for issues.
I think changing a bonus is a lot easier than changing a salary, especially if the bonus can be tied to the performance of the firm and therefore go down—salaries tend to be very sticky (ie hard to decrease)
I think it's a wash. If your company is performing poorly and needs to cut costs, eliminating a bonus will cost you staff. Lowering salary will also cost you staff.
This is exactly what happens. So then why not just include it in the salary at that point? All you are doing with bonuses is just causing an opportunity for issues.