When Tanks go Full Tank after Cleaver and One-Shot Carries

Claim is in the title, it is ridiculous for champions like Garen and Darius to build an early cleaver and full tank then one shot carries with just their ultimate abilities or just a simple two ability combination. In most other games tanks are meant to "tank damage" while the carries burst. The higher the devision, the more true this becomes. However, considering much of the player-base is not considered to be high elo I feel that being able to out dps a dps marksman is absurd for an almost full tank to be able to do. Especially when it is true damage like Darius' "Noxian Guillotine". When combined with Darius' passive "Hemorrhage: causing his target to bleed. This effect stacks up to five times. Enemies that reach max Hemorrhage cause Darius to gain Noxian Might, greatly increasing his damage." and add that to the statistics of his ultimate, "Leaps to target dealing 100/200/300 (+75% bonus Attack Damage); For each stack of Hemorrhage on the target, Noxian Guillotine deals an additional 20% damage." It just seems unhealthy to me for a tank to do over 300 true damage with one ability and have damage over time with the ability to out sustain. Who needs marksmen when you have an "ADC" in the top lane : ^) ?