So Why Can Shiv Crit Exactly?

Why do ADCs need burst? Their entire role is supposed to be consistent DPS, but now you have Vayne Cait and Trist getting 2 items and 1 shotting squishies while maintaining top-tier DPS. Burst mages are already irrelevant enough, but now a damn Tristana, the most ADC ADC in existence, can annihilate anything without sacrificing any DPS.