لَيْسَ كُلُّ قَدِيمٍ صَوَابًا، وَلَيْسَ كُلُّ جَدِيدٍ خَطَأً.
Not everything old is right, and not everything new is wrong.
Each clause negates a universal claim, rather than universally negating the predicate. Denying that everything old is correct does not mean nothing old is correct; the same applies to the claim about new things.
FormRequired reading
ليس كل رأي صحيحًاNot every opinion is right
لا رأيَ صحيحٌNo opinion at all is right
The crucial discipline is preserving the scope of negation rather than turning one structure into the other.