Plot is flavouring you add to a steak called "interaction" where it concerns role playing. It's best used to sprinkle some extra stuff on. Not to be the direction players use. The direction should be taken and driven by the players.
In plot terminology: You create the exposition, and then the players choose whether they want to pursue the rising action or abandon it to find a new thing to do.
In practical application: You have a zombie RP about hunkering down and surviving. Instead the players choose to go find fuel and a working plane/boat to cross the Pacific and get to an island. Maybe Hawaii. So they also start looking for naval charts and someone who has the knowledge of the seas, then partway through they find out fuel is useless so they choose to head far, far north instead, and start creating a convoy of farms in the south and housing in the north where most survivors go to hide, hoping that cold affects their joints, or...
See where I'm going with this?

A plot
can be used, it just doesn't have to be. In the same way you could use a plot for a video game, but you don't have to. You could just create a sandbox like Garrys Mod and let the modders and players create their own adventures, ideas, pursue their own goals... Last I checked GMod sold over 1,000,000 copies as an indie title. I'd say that formula works. And since RP's are more like games than literature... It works for RP's too.
Hell, pick up any copy of D&D. You could just simulate battles for no reason, or explore an open world with no plot, no conflicts no stories and go from there. Anything is possible. It's the one gem that truly puts it aside from literature or most gaming outlets; RPing is whatever you make of it, and the only thing you need to make RPing work is interaction with other people. Be it through nations, character, narrative... Anything. It's just that characters and nations are typically the most common way we express ourselves, or our imagination in role plays. So...
#1: ...Okay, so you're arguing... For what? That's literally what I stated. Plot is not required.
#2: ...Okay, here is where I think you are confusing plot for continuity. You can have exciting action sequences and what not arranged in a series of events, and not have a plot.
This is a plot. RP's can completely disregard it or throw it away at any time for a new one, or simply not have one. You can have battles... For no reason. And it's perfectly fine. Honest!
Now that we've clarified that you can do
any scene or series of scenes without a plot, IE without a plan, IE without rigid structure, everything you've argued here... Can be applied... Without a plot.
A plot by literary standards is merely "everything acts for the greater narrative." That's it. Characters, scenes, relationships, everything is enslaved to the narrative. It's optional. You don't need a plot to save the world in an RP. The characters don't have to act within a narrative. Everything I've done has repeatedly, and demonstrable proven to me, over and over, that a greater narrative only enslaves characters. It railroads them. It forces them to do as you please, instead of doing what is natural.
Plots are not, in any manner of speaking, whatsoever necessary for an RP to succeed. If they were, in any manner whatsoever, then Nation RP's as they presently exist?... Couldn't.
End of story.