A few issues with your argument I feel.
There has to be a point where you can go "Look, enough is enough" and deal with the situation. I would say that when EVERYBODY in any sort of position of knowledge is going "There's only one person in the area even capable of pulling shit like this", you have gotten well past that point.
So, the first bit on the rumors such as Monks not really being more viable party members than brawlers. The issue here is that for all it's obvious to the readers who are following the first monk, it's not actually an unreasonable view someone might consider. Monks are new, and there isn't all that much information about them at the moment. Many people likely are concerned that monks are just brawlers with a fresh coat of paint. So the rumors going around aren't really that unreasonable, and lead more to people taking a cautious, want to get proof it's not the case kind of approach to the matter. Brawlers who are interested of course just go to check it out, because they have nothing to lose really. Also, I'm not sure it is actually super obvious to most adventures where it's coming from. There were multiple clues that made it obvious to the protagonists, which included the rumors, but more strongly was based on the assassination attempt that was followed by the assassin strongly implying just who was paying for the job without actually stating it. To most, the rumors likely seem to have arisen pretty organically.
As for the assassination attempt, I'm pretty sure adventurers don't rise up over something so minor as a noble or the like taking issue with a single adventurer and trying to have them assassinated. For one thing there are adventurers who specifically facilitate that as a career.
Let's focus on the fact that the Duke is fucking with a World Boss defense!
Not sure where he's been doing that. Or at least if he has it's a recent enough occurrence that most likely haven't realized or are putting it on hold to deal with said world boss, rather than trying to start a war on two fronts. Keep in mind, the world boss is very recent news, and has attacked only a couple minor villages. There haven't even been any real conflicts with adventurers who are actually working in defense of it yet. I rather doubt the Duke was needed to start rumors that Samuel was lying about his fight with the boss. After all, he's a new fairly adventurer who is claiming to have fought a world boss in a first contact situation and lived to tell the tale. Scepticism as to the validity of the claim is entirely reasonable on the part of various other people planning to help.
If they know anything about fighting world bosses, his claim seems extremely far fetched.
More likely to be traced back to the Duke is the claim the boss showed up in response to the Monk title, but that's not really going to impact the defense against the world boss, it would just make people less fond of monks in the aftermath. Additionally, that really just feeds into peoples desire for a scapegoat to blame for problems. Particularly when the actual cause is less tangible than a person, or at least less reachable. It sucks, but it's not some big problem that will stop the world boss defense. Likely every time a world boss shows up you get a few dozen different rumors claiming its appearance was "caused" by some random factor that was involved in the discovery or just big news when it showed up, because the idea that there is something people can actively try dealing with to prevent world bosses coming is more comforting than it just being some uncontrollable random chance event. Much like how people pick up all kinds of random habits or items to place the blame on for bad or good luck in games of chance (be it something like gambling for money, just rolling dice for an RP, or trying to grind some rare drop in an RPG).
Given that the Duke has been skirting this line for years
Has he though? We know he runs his dukedom for a profit with some degree of incompetence, and is in deep with the local rogues guild, but that's not really pushing the boundary there. Sure, he made his dungeon town so that it's easier to get in there, and then you need to work hard to afford to leave, but he also made it fairly reasonably priced to stay there, and really it's an intro level town that consequently has a good rep for turning out competent adventurers because they had to actually get skilled enough to earn the coin needed to leave rather than just deciding to go to the next dungeon on the difficulty rating when they got bored of that one and then getting badly hurt/killed because they weren't actually ready.
Like, sure, one rumor would make sense catching and spreading among the population about the strange new class and its unknown leader. Two? Maybe. People do like to gossip. But at current count FIVE? All designed to paint them as EVIL as possible? That's clear and obvious enemy action.
Is it really? I mean Monks are huge news. There's bound to be far more than just five rumors about them, likely more in line with dozens or hundreds. This isn't just a new title popping up, this is a title advancing what was previously a dead end fighting style with only one title to its name, that couldn't even do well in parties due to the focus they had. If you couldn't find a half a dozen different rumors about them each day of the week in a random town where adventurers actually hang out I'd be surprised. There also are clearly at least a decent number of good rumors, but we don't see focus on them because they aren't really a problem for the protagonist and Monks as a whole. Additionally, even for all the way bad rumors seem to be sticking in the minds of people, most seem to take them with a grain of salt, adopting a wait and see approach rather than immediately buying into them. It seems more like those rumors are taking of mainly because the adventurers don't want to risk overlooking some flaw they were warned about in this new title everyone is talking about, so they keep it in mind and decide to test it for themselves, or at least find a reliable account noting it's not the case (which if they're going around asking about it will help to spread the rumor).
There is also the fact that for all Samuel and friends know the Duke is behind some of the stuff with the bad rep Monks are getting among gossip, he's hardly the only suspect. A lot of places have overflow dungeons that liked taking advantage of Brawlers having a lack of other prospects to get cheap labour preventing the overflow. Any number of those places could thus be contributing to the rumors about monks in hopes of discouraging the brawlers from leaving in search of a better, more exciting, career as a monk.
And because it bears repeating, the big clue that helped Samuel and everyone realize it was largely the Duke behind stuff was what the Assassin said, not the rumors. Most adventurers don't have that piece of information to make the connection, and in all honesty, we don't even know how accurate the view Samuel has taken of blaming everything bad people are saying about monks on the Duke is. It's entirely possible that there are a few dozen groups independently spreading malicious rumors about Monks for their own reasons/profit, and Samuel is just mistakenly laying responsibility for all of it at the Duke's feet because the Duke is the one who decided to hire an assassin.
owrtho
"? Hell, no, that doesn't make a lick of sense! This ought to be being treated with the seriousness of an atomic bomb! Because that's what the lore says they are-literal living country killers! But instead we get...this. Apathy in the face of serious breaches of the privilege afforded adventurers, World Bosses, and the potential loss of literally priceless information on where World Bosses come from.
