WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 26, "Traitor," now streaming on Funimation, Hulu and Crunchyroll.
The Rumbling has officially made landfall and the countdown for the extinction of the human race has begun in Attack on Titan. While the surviving members of the Scouts and Marley struggled to coalesce for their mission to save the world, the Colossus Titans have been moving (literally) full steam ahead. Given the elapsed time since the Rumbling began and the distance of the steam, Hange has concluded that the Rumbling has reached North-East Marley and the cities there have likely already been wiped out.
Even though the Colossus Titans crossed the sea quicker than anyone anticipated, the Scouts and Marleyans have still spent a detrimental amount of time airing their dirty laundry rather than carrying out their mission. The Rumbling has begun taking lives outside Paradis Island, and the only people with any intention of stopping it haven't even secured their means of transport to pursue Eren yet. This unlikely alliance (that really needs a name) has spent so much time debating the morals of their actions and the history of the war that Eren and his Colossus Titans now have a tremendous head start. They're going to have to get a move on, or else there won't be a world left to save by the time they catch up to Eren.
Attack on Titan has yet to show any of the destruction that the Rumbling is wreaking upon the world, only providing Hange's deduction that the Titans have surely reached Marley. Willy Tybur turned out to be right on the money at the beginning of Season 4 when he declared that there would be nothing the world could do to stop the Rumbling. Worse, the Marleyans have ignored the warnings of the Eldians in the Liberio Internment Zone who heard Eren's message about destroying the world. If Marley, the nation closest to Paradis Island and best equipped to kill Titans, is going to be blindsided by the arrival of the Rumbling, there's little hope for the rest of the world.
Despite the fact that all the Colossus Titans in and around Shiganshina departed Paradis Island in the same direction, what the anime depicted was only a small portion of the true size of the Rumbling. There were "millions upon millions" of Colossus Titans inside Paradis Island's Walls, meaning that a single file line to destroy the world is unlikely. The Rumbling would more logically occur in a single wave across the planet as the Titans set out from Paradis Island in all directions. This means it's highly possible that the North-East coast of Marley is not the only place where the Titans have made landfall by now.
Since there is no stopping the march of the Colossus Titans as they spread around the world, finding Eren is the only chance the Scouts and Marleyans have for stopping the Rumbling. Yelena may or may not know where Eren himself is heading with his own Founding Titan, and the fate of the world rests on whether she will stop playing games regarding the information. That even Onyankopon is only opposed to torturing Yelena on the grounds that it would take too long speaks volumes for how serious the situation has become now that the group has realized how far ahead of them Eren has gotten.
The Scouts and Marleyans softening their hatred for one another is great and all, but that they keep stopping for heart-to-heart conversations is becoming an issue as the death toll climbs every second. The vast majority of the world will have no idea that the Rumbling is coming or what it even is, meaning there's little to no chance that most of the human race will make it out of this alive even if Eren is stopped. The emotional catharsis of the Scouts and Marleyans won't mean much if they take too long to stop the Rumbling and the world becomes one giant graveyard.
