There's nothing wrong with 'plot' in a general sense (because that could really describe the connection of any two things the game, really, or any kind of cause and effect), and especially not for a one shot, which is what actually being discussed here. In general, I never plan for a game where X needs to happen or everything else goes bye-bye though, which is I think what some posters mean by 'plot' here.. I think we've mostly established that that can be problematic. For a one shot though, especially one played with strangers, you probably need strong sign posts to help the players navigate the scenario, and a good idea of what happens when they don't. That's not the same as 'the players must do X to continue though'.
A mistake I see a lot there is in scenarios that assume that only PC actions change things or set events on motion. What a silly idea. The PCs should have a lot of agency to influence, alter, and mitigate events, but they aren't the sole cause of them. As a mostly-GM, I find that as long as I have a good sense of what happens in either event of a PC choice then I can usually make the consequences work both ways. What I always try to do is make the consequences for PC choices appropriate and interesting.
To take an example from upstream, where a PC pulls a knife on an NPC to gain compliance, there all sorts of perfectly appropriate consequences there that don't match the stated PC goal. If that PC was doing some legit RP stuff, like maybe they are playing the heavy and a little light intimidation is in character, cool (although fingering the knife handle would have been more than enough there IMO). On the other hand, in the many instances where that could just me bad RP and really more of a player tantrum than actual RP, I might lean a lean harder into the consequences. Maybe the NPC screams loudly in fright, or screams and runs away (just to pick an example). It's tough to adjudicate in a vacuum, but what I'm getting at is that my job as GM (as I see it) is not to enable the players, but rather to fairly adjudicate their choices (mechanically or not) in terms of the current fictional state. So if a player makes poor decisions, or doesn't think things through, they are probably going to be disappointed in the results.
That said, I am also completely fine going the same way as
@FitzTheRuke and telling players in a stranger game to shape up or ship out. I'd rather boot someone than let them ruin things for everyone else. I'm a GM, not a babysitter or an edgelord enabler.