Why did you hire the rogue instead of a bard?
Because they're totally different classes with totally different functionalities and nearly no overlap? That is just a godawful example of "overlap" in 5E? Maybe in 3E that was a question. Bards have breadth of skills and are massively powerful casters in 5E. Rogues have more powerful, reliable skills in specific areas, and are high-single-target DPS, not powerful casters.
Bard and Sorcerer or something is a better overlap.
The monk fills the same basic role as a rogue. Monks are better at some things (speed, dealing with enemy casters, disabling a big foe) and worse at others (skills, raw DPR), but the two classes occupy similar niches.
They absolutely don't. Outside of combat, Monks are pretty close to objectively worse than Rogues. I mean I'd say they are objectively worse. The number of places a Rogue can be useful, and significantly useful, out of combat, is going to vastly exceed that of a Monk, with stuff like Reliable Talent (which is amazing) alone.
Inside combat, they're closer, but Monks aren't generally as good or versatile as Rogues. Most combat don't involve casters who need "taking out", and yet who a Monk can safely solo. This is big lie of "Monks are for killing casters". Monks don't have enough burst or survivability to actually reliably pull this off unless the enemy basically doesn't respond, and can't hit and fade like Rogues can (nor shoot from the shadows). A Monk has to run up to his target, and then spend multiple rounds taking it down whilst somehow not getting beaten to death. Doing this even once in a combat blows pretty much all their per-short-rest Ki at lower levels, and they can maybe do it twice per short rest at mid levels.
But even that is still not addressing the lie - which is that killing enemy casters by penetrating their lines with a specialist class is routine requirement in 5E. It isn't. Loads of situations obviate it:
1) Your side has good ranged attacks.
If so, chances are, you can just gun down the caster.
2) Caster isn't actually a significant threat.
This is very often the case in 5E. PCs are very often spread out, and enemy casters often don't have that great of a spell set, and often are lower-level than than the PCs.
3) Caster has too many HP (or other defences) for this to work.
This is often the case in 5E, due to HP inflation. A lot of NPC casters are basically as tough as the front line. A solo Monk is unlikely to be able to take them down, and thanks to 5E's OA rules, can't even stop them casting freely (I guess he makes ranged attack roll spells have disadvantage? like a familiar would?)
In fact, in my experience, the percentage of casters who are both easy enough to kill to make this a valid strategy, and dangerous enough for it to be worth it is pretty tiny.
4) Enemy line is not coherent enough to require specialist penetrator.
I.e. the Barbarian or whoever can just mosey on through the wide gaps between enemies (esp. after dropping an enemy) and kill the caster.
5) Caster doesn't immediately get CC'd.
Again, this happens a lot in 5E. A couple of failed saves from the caster are often enough.
6) Combat lasts long enough that the caster doesn't essentially die at the same point anyway.
The average length of combat in 5E, by design, is 3 rounds. My experience is reality tends to go 4, but YMMV. Often a Monk takes 3 rounds to even kill one caster (I mean, maybe the Monks I've seen are just bads but I doubt it). As casters can basically free-cast with the Monk there, it doesn't matter much if one Monk spends three rounds grinding him down, or multiple people attack and kill him on round 3/4. Stunning Strike is the answer to stopping casting, but that's more Ki to blow, and doesn't always work, and is basically the only thing Monks really have going for them.
It's a false role.
