If you need the job done and because we are not resource constrained you are going to have to do something to make that job attractive. Maybe even address the things that make the job shitty or even do the work to automate it so the janitor can become a dancer like he always dreamed of.
You’ll just be paid low wages in the new resource.
But regardless, we’re still far and away from “let everyone do only what they wish”; sure everyone might survive (due to presumed abundance of survival resources ), but to do as you wish for any job reliant on others, you’ll need to coerce others to do things they don’t strictly want to do.
And so we have scarcity, and politic.
The resource-abundance means the janitor can also optionally do nothing — he doesn’t necessarily need to janitor against his will — but it doesn’t mean he can become a dancer (that anyone will watch). He can become a dancer only so far as one might do alone, in their room, but to be trained, or to do it in the professional arenas, or to be watched, etc requires the coercion of others, with requires him to be sufficiently wealthy in something valuable, which must be scarce, and naturally unevenly distributed.
And he might janitor to save up to be a dancer.
Of course, you could always pretend to have an audience in a VR simulation, or an army of robots, but that’s perhaps a fairly pathetic fulfillment of his dream.