(Excuse me while I bring it back to the OP)
Well there's more than one way to "tank," right?
Usually when people think of a "tank" character, they think of someone who soaks up the enemy's attacks, spells, actions, damage, whatever, so that the rest of the party doesn't have to. And there are a few ways this can be done.
One way is to soak up damage that would normally be applied to another, squishier character in the group. This is what your high-Con barbarians and fighters do...they absorb damage so that the cleric and sorcerer don't have to. It's the classic definition of a "tank" for a reason. (Barbarians are my favorite because not only do they have high hit points, they also take half-damage when raging. That's a two-for-one special.) Unfortunately it has a drawback: those hit points eventually run out, and need to be replaced...which can be a drain on the party's resources.
But another way to be a "tank" is to make the opponent waste resources. This is what your abjuration wizards, bards, and anti-magic mages do...they "tank" by cancelling out incoming enemy magic before it can be used against the party, effectively making the enemy "waste" their actions and their spell slots. It's not as common, and not as useful, but it's impressive when used against the right enemies. (Abjurers are my favorite for this tactic...they get temporary hit points every time they cast an Abjuration spell, so they get tougher the more they counterspell. Another two-for-one special. They're one of the few wizards who can "heal" themselves.) It also has a drawback: those spell slots run out quickly as well, and the party needs to rest more often to recover them.
And another way is to make the opponent waste their action economy. And that's what the monk and rogue do best...they provoke their opponent into making their attacks against them, which they will most often miss thanks to that "bonus action: dodge" ability. Any attack that targets them is an attack that isn't targeting the cleric or sorcerer...and on top of that, it's also more likely to miss. (Monks are my favorite for this, because they have a defense for almost everything. They can dodge as a bonus action, sure, but they can also block ranged attacks, shrug off mental effects, take half damage from AoE spells and effects, they're immune to poison and disease...) The monk will quickly run out of ki points, but the rogue can do this tactic all day long.
So yeah. I think that Monks can make excellent tanks.
However, I think the best way to build a "tank" character is to stop thinking of it in terms of a character's identity, and more in the terms of a tactic that everyone should be able to do in some degree. When the chips are down and you're on the ropes, your character--regardless of what their dominant role in the group might be--will need a way to roll with the punches for a few rounds until the healer or the striker can get to them. Every class should have a "shields up, red alert!" in their playbook for situations where you need to hold on for dear life. I'd choose versatility over specialization any day of the week. (But I'm starting to drift off-topic again.)