People really, truly, fucking desperately NEED, I repeat FUCKING NEED to learn what the sub classes are for fuck's sake.
A juggernaut is a melee carry. They are designed to do as much damage as physically possible to the enemy team, nothing else. They are not tanks, they are not off tanks. They have bulk because their low target accessibility and even lower threat range necessitate some form of defense against AoE, ground targeted effects, multi-hit/pass through skills, as well as survivng being in range of (or focused by) up to the entire other team.
They don't give a fuck if you're about to be nailed by CC and skillshots as an out of position mage or ADC, it simply is NOT, AT ALL their job to save you from your own stupidity. That's on Warden and Vanguards (y'know, the actual defensive classes, or tanks, designed to take focus and punishment for their team while also either peeling or engaging with hard CCs). You need defensive characters on each team.
You don't necessarily require a tank, as enchanters can also offer defensive utility as can many mages-- stacking several of these champions, say a Lulu, and Oriana can then make a non-tank reasonably tanky temporarily so as to act as a psuedo tank (we saw this in Season 5 professional play quite a lot in the hyper tank meta, where non-hyper tanks would be shielded and ulted and used as a delivery system for the AoE displacement and knock-up from those aforementioned champion's to simulate the effect a true tank would bring to the table) --but note that it takes 3+ champions to adequately fulfil that role, verses picking 1 actual tank and them just doing their thing. Bonus is you get way more damage this way, and tend to have better scalings come lategame, but it requires that those three at least stick together and communicate well.
In a perfectly balanced game of League of Legends, a juggernaut could replace your ADC as your main carry. So would a skirmisher (think Yasuo, Riven, Fiora, Jax, Cam) or a burst mage. They are all carry archetypes that are focused almost entirely upon damage and only secondarily on providing the tools those champions need to survive while providing the damage that they can.