The thing is, the Monk doesn't HAVE the option to invest more into Damage. It has no GWM feat or PAM feat, and barely any magic item support. There's nowhere to go with your Monk's damage.
Okay, but is it the fault of the monk that they received little to no support in feats and magic items (two "optional" sets of rules that are completely under DM control?)
And, I'd argue that they get the major benefit of PAM (a free bonus action attack with mod) at level 1.
But if the issue is "there are no feats or magic items designed to make this class better" (By the way makes can use shortswords, so they also have access to just about every magic sword a fighter does) then that can be fixed by designing new feats and magic items. Like Crusher, which is some additional very nice control for a monk character.
That Greatsword Fighter only needs to invest in STR to have that AC... The same stat he's using to attack with. Then his second highest stat is probably gonna be CON. Couple that CON with the d10 Hit Die AND the Second Wind feature and the Fighter has a signficantly greater pool of HP to soak up damage with. And then that Fighter can pick whatever he wants as a third stat! Want to be INT focus? CHA focus? WIS focus? Pick whatever... The Monk is railroaded into DEX and WIS and probably will want CON in third place otherwise, to quote someone up there, they fall off a cliff.
Sure, Monks need wisdom. But, if they invest in Dexteritity, that is also the same stat they use to attack with and is boosting their AC. In fact, with a 16/16 start, A monk only investing in dexterity can reach AC 18. Only investing in their attack stat. And there is nothing wrong with Focusing Dex and Wisdom, those are two of the best stats in the game. Pigeon-holed? Sure, but it is a strong place to be regardless.
Yes, they have less Hp than the fighter. Not going to deny that. But investing in their damage is also investing in their AC. And your point wasn't "they have low hp" it was that they cannot match the at-will out-put of a fighter... But they can.
Maybe
@Helldritch is right, and it is only the perception that they are weak if they can't use their bonus abilities, but that perception is not reality.
The fighter offers more build option, despite not being as flexible at the table, than the Monk, and its subclasses are generally more solid too (some of them are awfully boring though).
The Monk does have the advantage of better saves than the Fighter, but the Fighter gets more Feat with which to mitigate any potential weakness (let's all be honest, you only need 3 good saves, the other three are basically ribbon).
I feel like the Monk feels too much like a Warlock and not enough like an Arcane Trickster or an Eldritch Knight. Their at-will core should be stronger than what it is now.
The Ki options are generally good, but if you take a subclass that gives you more options to spend on (but no extra ressource to spend on them), it becomes incredibly HARD to manage your points effectively.
And for a class that's presented as being all about punching stuff and using your body as a deadly weapon, I feel like it's got a very narrow range of optimal applicaton and a very high skill floor that feels at odd with the fluff.
I'm not sure if the Fighter really offers more build options, but I think you are missing a point with the Monk.
Most of their subclasses don't spend more Ki, they instead empower the Monk's Flurry of Blows (open hand and drunken master specifically). Shadow does spend Ki, but does so outside of combat generally for very powerful effects.
But the point I keep trying to make is that the monk without Ki, before level 11, is not falling behind, unless you are dealing with a -5/+10 feat.
Don't forget that at 8th level, the greatsword fighter got his third ASI. Since he's not using feats (otherwise he'd grab GWM), he already maxed out his Str at 6th level, so ASI #3 went into Con. The sort of threat the fighter can stand against, the monk is using his ki and movement and bonus action to keep pulling away from, lest he die. Which is fine from one perspective; the monk can do that to keep up, and he can use the ki other ways in different fights.
But the big thing here is that the defense of the monk comes down to, "Monks are fine through Tier 2, if you don't use feats or magic items". That, to me, sure looks like an admission that "Monks lag a bit in Tier 2, unless you're fighting single-BBEGs to stunlock."
Most of the "without magic items" discussions are because the fighter is being given +1 Plate Mail and +1 Shields, or sometimes +2 on each. Meanwhile the monk is given nothing.
But, Bracers of Defense are a Shield for Monks, they can wear rings and cloaks of protection. They have magic items that increase their AC too. But, because the designers wanted +3 Plate Mail, but didn't want to stack higher than +3 Armor and +3 Shield, they didn't design +3 Rings of Protection.
Frankly, I'd remove the Attunement requirement for Bracers of Defense and make them uncommon (they are literally just a shield for people who can't use shields) and design some new items that are monk specific.
But, even with current magic items, a Monk with a Defender Shortsword, Bracers of Defense, A Ring of Protection and a Cloak of Protection, with 20/16 Dex/Wis can have 25 AC every single round. Very similiar to the Fighter with +3 Platemail and +3 Shield (26)
People just want the monk to never have any equipment at all, for whatever reason.
So this is where we have to look at the notion that “not all levels are created equal”.
your point is true at 1st level, but 1st level is intentionally designed to be moved through quickly, and many games start at higher levels.
Then from levels 2-7 the monk has lower AC. This is the “sweet spot” of gaming, the vast majority of games and tables are playing in this level range.
then at 8th level the monk equalizes (assuming specific stat bumps...and the fighter is not taking feats that boost AC...which is generally a reasonable if not rigid assumption). However for many campaigns this is epilogue time...the game is starting to wind down.
So for the vast majority of “screen time” at the table, the monk has worse AC.
Huh?
What kind of Rogue is getting better than 16 AC by level 2? Are all of your fighters suddenly in plate-mail before level 4?
This seems to be a very strong claim over a very broad range with no supporting facts.
Edit: Seems this is addressed.
You mention healing though, which brings me to another point. Much like the Rogue, the Monk should rarely be in a situation where they can be hit. Both Open Hand and Drunken Master give you ways to stay mobile when using Flurry of Blows, Shadow has their teleportation, ect.
In fact, I think Open Hand really highlights the issue with ignoring subclasses. They have the ability to use their Flurry of Blows for more damage, and control, and it allows them to freely disengage with the enemy. Then, they get the only monk healing ability, covering the other issue you see.
Yes they do, and a really good one. Since you're talking feats, my aforementioned shadow monk? I took the magic initiate feat and got hex. Hex with a monk is crazy good, for obvious reasons.
Yes, the only problem with using hex or hunters mark (I thought about getting it via the Fey-Touched feat before) is the heavy bonus action requirement, but if you need to dish out the damage, adding a single d6 to all four attacks is
huge
