WARNING: The following contains spoilers for Chapter #136 of Attack on Titan, "Devote Your Hearts," by Hajime Isayama, Dezi Sienty and Alex Ko Ransom, now available in English from Kodansha.
With all of the world-ending carnage currently gripping the Attack on Titan manga, it's easy to lose track of the series' smaller, expositional moments -- especially when they come from the mouths of Hajime Isayama's least-liked characters. In Chapter #136, "Devote Your Hearts," as the Eldian and Marleyan allies plot their next attack on Eren's Founding Titan, Gabi Braun makes an interesting assertation about the "true power" of the Titans -- one that could be the key to the creatures' eradication once and for all.
Her observation links to Eren Jeager's apparent resurrection at the climax of the War for Paradis arc. During this desperate scramble, members of the invading Marleyan military and defending Eldian Scouts found themselves united by one crucial cause: stopping the two Jeager brothers from making contact. If Eren, who possesses the Founding Titan, touched his half-brother Zeke, the latter's royal blood (from his mother, Dina Fritz) would unlock the Titans' full potential, as witnessed when Eren kissed Queen Historia's hand and inadvertently tapped into her family's memories.
Using a rifle, Gabi almost stopped this fateful encounter from coming to pass by shooting Eren's head off as he dashed to meet Zeke. But, as the Eren's head fell into Zeke's waiting hands, a spinal cord shot from his body, reattaching it to the head and bringing forth the Founding Titan. Using its full power, Eren began the Rumbling.
As we know from Attack on Titan's rendering of Ymir, the mother of all Titans' origin story, the dying girl was granted the power of these colossal creatures 2,000 years ago by way of a spinal cord attaching itself to her, which Eren Kruger refers to as "the source of all living matter." (This is, incidentally, probably why all Titans are vulnerable at the nape of their neck and why this position is essentially the 'cockpit' for their human hosts.) In Chapter #136, Gabi surmises that the "shining" spine she saw snake its way out of Eren to bring him back to life is the Founding Titan. "If that's the true nature of the power of the Titans, we might see it again if we decapitate him."
Cutting Titans at their nape is only a temporary 'death.' No one has ever found a way to completely stop the beings for good -- and it's unlikely that any one group would ever simply allow the power to lie dormant forever as King Karl Fritz commanded of the Founding Titan. The fact is, as long as Titans exist, Attack on Titan's world will never know true peace. Gabi's idea to draw this spine out of Eren's body again presents the very real possibility of ending the beasts' lineage for good. The Founding Titan, after all, is the cloth from which all other Titans were cut. Should it be destroyed for good -- at its spinal heart -- every other Titan may die with it. Hammering this connection home is the fact that all Eldians and Titans are spiritually linked via the Path, which itself resembles a cross between the Norse concept of Yggdrasil (the world tree) and a humanoid spine.
To summarize: If Eren is decapitated again, the Founding Titan's spine may reappear to save its host. If that spine is destroyed, the root cause of the Titan "curse" may be lifted for good. The pieces for this are already in place -- the explosives Pieck Finger tried to detonate at the Founding Titan's neck in the last chapter. Bringing down the Founding Titan will leave Eren exposed (his Attack and War Hammer powers notwithstanding) but whether or not Mikasa, in particular, will have the strength to cleave head from body once more is another matter entirely. However fans may feel about Gabi, she could well have the instincts needed to save the world.