"Worst that could happen" is usually used in a relative context though.
But in an absolute context, it's probably dependent on on who the subject is.  The worst thing that could happen to someone may not be the same thing for someone else.  You may have a worst thing, but beyond a certain point, it probably won't matter to you - in which case, you've reached your "maximum".  That's assuming you have a maximum degree of care.

If there's no subjectivity, well, wee don't have an objective method of comparing events...
Which is worse - being slapped in the left cheek or on the right?