SuttaCentral
Suppose, as a thought experiment, that every living human being attained perfect awakening. They all achieved the goal, reached nibbana, became Arahants. I know this is an extremely implausible scenario, to say the least, but just go with it.
How would they then live? What would they do? How would they sustain themselves? What purposes would they form or retain?
They would probably have to start producing food, otherwise they would starve.
Apart from that - it would be last generation of humans, no more reproduction and no more being born to this world, quite literally 
Intriguing! I donât know the intended direction of the question, but Iâm very fond of it from a social model perspective.
As an off the top of my head answer to the purposes aspect of the Q - I guess they might set about creating conditions for other beings, perhaps most immediately animals, to finding their ways to putting an end to suffering.
So no more reproduction then. I assume that is the case, but is that because fully enlightened beings are simply incapable of actuating the sensual desire that is a mental and physical precondition for copulation? And does that mean no dying beings of other species would be able to be reborn as humans?
I donât really have an intended direction. It is just something I was thinking about this morning while I was snowblowing my driveway, and I really donât know the answer. Iâm guessing people have a variety different ideas.
I remember I once read about the case of pacekka buddhas who after attaining to silent awakening would go to an isolated place and then âdisappearâ. They would simply lay on the rock and allow death to come. Is it something we find in any sutta or more of a commentary tradition? Maybe linked to the MN116?
Well⌠I donât know⌠But the way I fantasize about awakening is exactly this: there not being anything left to take up and go on for/from, instead of fearing death there is only room for full embracing of it, in a peaceful and definitive way ⌠câest fini
P.S.: One of the small joys I have when meditating outdoors and hearing sounds of animals going about their business and trees swinging touched by the wind nearby is to imagine how much more beautiful would the world be not being there anyone to hear, see or feel it⌠A world empty of me sounds much happier than the world with me in it! 
Interesting. The Pali Canon says that after the Buddha attained awakening, but before deciding to teach, he spent some weeks enjoying the bliss of release. Does it say whether he was eating during that time. I assume not, but really donât know.
Also, as I understand it, the awakened person no longer has any personal craving for future states of existence, but still has compassion. So does that mean, as Aminah suggests, they would want to sustain their lives for the sake of assisting other suffering beings - specifically, animals?
Ok, so after they have freed all of those animals in captivity - e.g. emptied all zoos and open all gates of animal farms, etc - what else could be done? Would it be justifiable to, for example, start a global initiative to erase all traces of humankind - e.g. demolish all cities, clean up all the mess we have so far made - and develop technologies to revert Earth to what it was 100,000 - 50,000 years ago? Crazy experiment indeed sir! 
Is there a way of teaching dhamma to animals? Obviously, one canât give them verbal lessons, but is there some way of conveying the spirit to them and helping them to have more peaceful hearts?
Good question. In the suttas we learn of the case of an elephant who attended the Buddha when he went for a solitary retreat at the foot of BhaddasÄla in the Rakkhitavanasaášá¸a, near PÄrileyyaka - it seems that the specific texts linked to this tale are Ja488 , SN22.81, Ud4.5, MN48 and Pi-tv-kd10.
Source: The Honey-offering Festival: Commemorating the Service of Animals to the Buddha
Maybe he had taught them something, or at least giving them the opportunity to manifest kindness and generosity would be of much help already⌠This actually makes me wonder whether we are all somehow linked to those very animals!. 
Yes, since at the very least the remaining humans would be leading much simpler lives, what would they do with all the dangerous garbage and useless edifices from the old craving-based society that would still be lying around everywhere?
Count on me to reforest the world and demolish it all (in a way that no animal is hurt, of course!). Working with this NGO could be a good way to get ourselves ready
http://www.menofthetrees.com.au/
Yes, in the traditional Madhu Purnima story about the Madhu Purnima retreat to the Parilleyaka forest the Buddha seems to be able to teach the monkey something about respecting living beings.
It seems that the specific texts linked to this tale are Ja488 , SN22.81, Ud4.5, MN48 and Pi-tv-kd10.
Well I suppose that you could copulate without desire (for copulation) if you were intent on preserving humanity - but can you do that without any, even slightest desire? I suppose Arahants would have no intent on preserving human kind.
As for other species⌠well, here we have some biological constraints. It would probably take few hundred thousands years to evolve something human-like again, if it would happen at all.
Firstly, with reference to [MN 116] I find it astounding and encouraging that so many individuals found awakening in that place âby themselvesâ, and so as pacekka buddhas, without any guidance of the Buddha-Dhamma.
In answer to DKervickâs question, perhaps supremely compassionate acts would be their guiding principle. Unifying speech, perhaps? Promoting understanding between those set apart by their views or their history. On a more âdown-to-earthâ level, they would be a model of compassion and wisdom to those seeking relief from the dukkha of the world. If they sought to remain in the world, would they engage with the madness of humanity or withdraw in to seclusion?
Either way, I wonder where they are!?
Iâm assuming as part of the thought experiment that all human beings are enlightened, so there is no longer any madness of humanity to escape from. But if people are no longer living in a mad way, in what way would they live instead?
Ah indeed. Maybe a utopia scenario? Thatâs assuming no babies are born from the point of mass awakening. Unless we can have enlightened babies. Society would cease to exist as we know it and no-one would support the awakened onesâŚwhy should they continue to live?
Well Ajahn Brahm notes:
There are nine things that an arahant, by nature, cannot do: store up possessions, intentionally kill any form of life, steal, perform sexual intercourse, tell a deliberate lie, and act improperly out of desire, out of ill will, out of delusion, or out of fear (AN IX,7). For instance, since sensory desire has been totally transcended, there is no spark left to ignite the passion for sex. All arahants are âpotently impotent.â
But I suppose they might employ technological means.
