To my understanding, which may be flawed, isn't the monk a midliner class?
What I mean is there are primarily three types of classes: Frontliners, midliners, and backliners.
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Frontliners are face to face with enemies, typically "tank" types with d10 hit dice and heavy armor.
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Midliners are often "striker" types like the rogue and the monk, that move in to make a strike(s), then move out of the creatures reach. They typically have d8 hit dice and only use light armor (or no armor for monks).
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Backliners are typically the "blaster" spellcasters and ranged classes that have no intention of being in close combat, like wizards. d6 hit dice and little to no armor prof's.
Of course a few standout classes don't fit this model (like the bard), but it holds true for most classes.
Edit: Not that people actually play classes as they are designed