Logic to decrement by one
Have you considered adders? You can subtract with an adder.

You just need to add -1 to the input value. A pair of 7483 4-bit full adders should handle your 8-bit input. Apply the input X to the 'A' inputs, and apply the binary representation of -1 to the 'B' inputs. Fortunately, the 2's complement representation of -1 is all ones; this means that if you just tie the 'B' inputs all high, you're done. What comes out should be X-1.