The big problem with twists like these...

If you know they're coming, it sort of ruins the surprise. If the GM asks if it's okay to have party betrayal (or if someone else asks and the GM says yes) then you're constantly on the lookout for it - because why would they ask if it was irrelevant? Of course, nothing says the GM can't ask an irrelevant question in the same manner they keep irrelevant minis next to their screen, but it's something that's usually frowned upon (what amounts to non-consensual PVP), so if it's known to be ok, you'll be looking out for it and then the twist won't stick.

Of course, if you don't know it's coming, then it's never a place your brain will go. You aren't just going to accuse a character (and thus player) of working against the party because that's a heavy accusation. It carries a lot of weight behind it since you're only a few steps down from calling someone a problem player. Players often don't have a good enough grasp on other players' characters to notice behavioral shifts, and players often don't have good enough acting skills to roleplay them correctly.

I've yet to hear a story where someone figured this kind of twist out before the reveal, and that doesn't surprise me at all.

But by no longer utilizing poison against the party because of the monk, the monk has effectively made the entire party immune to poison by virtue of it no longer being present in encounters! Hah!

But seriously though, cutting out stuff you know the party will hard-counter is just going to make the party not feel as cool. A balance of both is important. Believe me, as the guy in the party who could cast Silence, I know; hard-countering every boss encounter kind of makes the boss feel lame instead of fun.

Not quite the same, but a Paladin in a campaign I was in once bought a Shield of Missile Attraction for cheap because the shopkeep thought it was cursed.

I remember offhand it being 50 GP per level of the spell you want to cast, though I can't say where in the PHB I read that.

It's 3000 GP just for the material components, plus another 400 to pay the caster. At one gold piece a day (the amount a skilled artisan earns) it'd take 11.5 years to earn a clone with a poor lifestyle (2 SP per day).

So you're living a poor lifestyle for basically half your professional life, just to earn the ability to repeat your professional life and spend another 11.5 years of it earning the ability to repeat your professional life just to spend 11.5 years of it earning the ability to... you get the idea. You'd also need to find a caster capable of casting an 8th level spell, which is rare.

Possible? Yes. Popular? I doubt it.

A clearer way to phrase it might be "there are no rules for the genre of fantasy". An individual world needs self-contained rules, yes, but just because Tolkien's Dwarves have beards regardless of gender doesn't mean that your Dwarves need to be the same.

Wild magic Sorcerer: I do not control the Lobsters (they just kind of showed up)

It's just an analogy. Here; let me try one more time.

If you're playing a horde shooter and your friend reveals they can just spawn a boss on top of you at any time, it kind of kills your desire to keep playing - at least with them.

No offense, but you seem overly fixated on all the wrong things.

Whether it ends in a TPK isn't relevant. If you're playing capture the flag and your opponent reveals they can just teleport your flag to their base it'll have roughly the same effect. If the GM can just say "you lose now" it'll seriously demotivate anyone who is trying to enjoy the game, for whatever reason.

Overall, the difference between having an in-character "please stop being murderhobos" moment and having an out-of-character "please stop being murderhobos" moment comes down to how likely it is for the players to take the message to heart. If it's just some dude that's telling them to stop being murderhobos and is an unwinnable fight if the players refuse, that sets a distinctly different tone than the GM pausing things for a moment to explain the current situation to the players.

Both can work, but keeping it as a narrative element has a higher chance of failure, since it's possible the players could interpret this as just another NPC encounter instead of the GM's thinly veiled wishes for the future of the table.

Overall, the only people who care about the story are the people at the table, and having a moment of jarring change in the characters to set the narrative back on track is fine. You'd probably want to do something like that anyway to paper over the past behavior, otherwise the players could listen to you and be understanding of what you want, and still get punished for the stuff they've already done.

That's better communicated through... communication.

I don't know about you, but if I were playing a game to win and my "opponent" reveals that they can just cheat and instakill me whenever they feel like, I'm more likely to just stop playing the game than to try to play it for fun. Even if I did try to play it for fun, it would be hard to really enjoy it when I know that any encounter can just be a big middle finger.

If you don't explicitly tell people what they're doing wrong and how to fix it, it's unlikely that they'll figure it out on their own.

You check the label and realize it actually says "Tenser's floating Dikc", but the salesman is already gone.

In my eyes, the Rule of Cool is best used as the opposite of the Air Bud Clause. (For those who don't know; the "Air Bud Clause" refers to a rule in basketball that basically says "it's not allowed just because there's no rule against it".) TTRPGs are imperfect systems, and you are going to run into a scenario that isn't covered in the rules. Rule of Cool is best used here, rather than to bypass rules that do exist.

But also; some systems can be really crunchy, and a lot of the time it can be more fun for everyone involved if you just say "you know what, that's cool, let's do it" than to pause for five minutes to leaf through some rulebook (because seriously; you can't always know the entire rulebook by heart) trying to determine if and why they can't.

Of course, doing this too much is dangerous. Hence "in moderation".