Malphite doesn't need to stay alive to cause havoc for the opponent's ADC when the ADC is destroyed in three hits. That's the entire point behind AD Malphite.
Find the 12/5 Malphite game (while it's still in my match history). With that build, MF quit existing when I said she did. She was dead before she hit the ground from my ult. Even with a Warmogs, she'd have been hard pressed to survive three 1.2k damage crits in the span of maybe 2 seconds.
Gimmick builds work sometimes. That is why its a gimmick build...