What will be the situation of the world when every beings will attain Nirvana?

The lotus cannot grow without the mud, the mud would be meaningless without the lotus. Thus the mud and the lotus are the same, they condition each other.

Ajahn Chah:

Nibbāna is found in samsāra. Enlightenment and delusion (samsāra) exist in the same place, just as do hot and cold. It's hot where it was cold and cold where it was hot. When heat arises, the coolness disappears, and when there is coolness, there's no more heat. In this way Nibbāna and samsāra are the same.

Prajñāpāramitā Hṛdaya sūtra:

« There is no suffering, no cause of suffering, no end to suffering, no path to follow »

Zen Master Ryōkan Taigu

Where beauty is, then there is ugliness;
where right is, also there is wrong.
Knowledge and ignorance are interdependent;
delusion and enlightenment condition each other.
Since olden times it has been so.
How could it be otherwise now?
Wanting to get rid of one and grab the other
is merely realizing a scene of stupidity.
Even if you speak of the wonder of it all,
how do you deal with each thing changing?

According to Ven. Ajaan Tong, there are four things without limit: The knowledge of a Buddha, the world of beings, the universe, and space. The idea of a limited reality is more of a western concept, tied in with the Abrahamic religions which purport a creation and end. If anyone has any reference to the "four things without limit" that the Venerable sir is talking about, that'd be great. I have no doubt he is quoting a Theravada text, I just don't know which.

With that being said, as far as Buddhism is concerned, even if Samsara were "depleted" of beings, it would simply cease to exist altogether, as it arises due to supporting conditions. If you remove those supporting conditions, it no longer arises. And Buddhism views samsara as devoid of any useful or beneficial intrinsic qualities, so in any case it is of no matter.

The Buddha explicitly declined to answer:

• Whether or not the cosmos is eternal
• Whether or not the cosmos is infinite
• Whether or not the Tathagata exists after death

Cula-Malunkyovada Sutta

And why are they undeclared by me? Because they are not connected with the goal, are not fundamental to the holy life. They do not lead to disenchantment, dispassion, cessation, calming, direct knowledge, self-awakening, Unbinding. That's why they are undeclared by me.

He seems to imply that both views are wrong view (or at least, not "right" view):

It's not the case that when there is the view, 'The cosmos is finite,' there is the living of the holy life. And it's not the case that when there is the view, 'The cosmos is infinite,' there is the living of the holy life. When there is the view, 'The cosmos is finite,' and when there is the view, 'The cosmos is infinite,' there is still the birth, there is the aging, there is the death, there is the sorrow, lamentation, pain, despair, & distress whose destruction I make known right in the here & now.

If not, where is the fallacy in the above argument?

If you don't mind me saying so, a fallacy seems to be to be that you're postulating something good (i.e. "cessation of suffering"); and then proposing your attachment to and your imagination of something (i.e. "the picture of the world" and "the entire world" and "a land of the dead" and "a desirable goal") as a counter-argument; for example the following are meant as some absurd examples:

• Suppose nobody needed to die anymore: how could grave-diggers then earn their living?
• Suppose everyone's basic needs were magically taken care of, with enough food and shelter and protection from violence for everyone: society would collapse! Who would go to their jobs anymore? We'd be living in a world without slaves and without armies.