Why not just say, "Rocks fall; you die"?
Because that's low-quality storytelling. Establishing that a story element CAN be told badly, is not the same as establishing that a story element can ONLY be told badly. The OP included an explicit caveat about whether a pro-active mastermind villain would be fun for the players; why are you disregarding that?
I provided an example in the OP to clarify my intent. That example (a) was nonlethal, a wild goose chase, it ends whenever the PCs decide they're done; (b) involved an NPC *approaching* the party to tell them about the Sand-Burrowing Fortress and give them a map; (c) included an NPC "rescue" pre-scripted; (d) included a clue for later use (the map; it was drawn by the villain, and now the PCs have a handwriting sample). When an NPC initiates contact, saying "Hm, I look this guy over" or asking some confirmation questions is NOT the same as making Perception and Insight checks for every one of the dozens of NPCs in the tavern's dining room.
Yes, a perfect assassination is like rocks that you can't dodge. A perfect assassination is also only possible AFTER the villain knows who the PCs are, where they stay during downtime, etc. Villains should not just have that info by fiat; NPCs don't automatically know everything the DM knows.
In an RPG, a LETHAL attempt should have both a way for the PCs to spot/avoid it (in which case, they get XP) or a way that it goes awry (no XP, maybe they owe an NPC a favor now, maybe they suffer nonlethal harm... or at worst a reversible death). The attempt should also be an opportunity for clues.
In fiction, sometimes the audience, but not the heroes, sees the beginning of villainous sneaky plot. For example, the assassin in "Raiders of the Lost Ark" with a trained monkey. IMO, the moment when Indiana is about to eat a poisoned date is great drama. Tension happens because of the difference between what the audience knows and what the protagonist knows, and is resolved either when the hero discovers the threat by their own actions (yay competent hero), or is saved by good luck or helpful NPCs (as in the previous example). Perhaps one PC gets a clue in time to rescue another; for example, what if the assassin knew about four of the PCs... but not the fifth, who happens to be in the room, when the assassin thinks the target is isolated and alone? Or maybe the target is alone, but the wizard's cat familiar is in the room, sitting by the nice warm fireplace?