Well according to Riot definition of assasins apart from killing single person in 1 rotation with high burst damage, they have an dinsengage tool to get out of the situation ( see zed R, talon E, lb W and RW etc). The only exception to that rule is Rengar who hasnt any jump away ability and have to use his empowered W or walk away to leave the fight.
AP Malph, Galio is similar to full lethality J4 ( full pen J4 before lethlity came out) or diana or nocturne or full lethality Wukong. They are called divers. They dive in deal absurd amount of damage to single or multiple enemies but they dont have any escape or cc immunity to help them bail out. They stay and fight and die ( or kill everyone around them 1st).
As for the damage creep, I agree-its pointless to build tanky since you will die within 2-4 seconds regardless of building def or attacking. They should buff tanks HP and resistances and nerf by 10 the damage across all items and abilities.