Peter and Ace at their instrumental peak, which was not spectacular, but serviceable in the KISS context.
Then booze, drugs, egos, and laziness (certainly in Ace’s case), began to rot not only their playing, but also the band cohesion and dynamic.
Say what you want about Gene and Paul, but without these two, KISS would never have been.
Of course, the fact the group has continued to exist past the later-nineties (when, by any measure, they should have “left the party”) is strictly both mens’ fault, so there’s that.