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.
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.