To answer your question.. we need to look at the timeline (you can find it in http://accords-library.com)

after the game events 3473, some Gestalts were still able to fuse back with their corresponding replicant shells. However the Grimoire NieR book concludes with this statement:

“"Project Gestalt" proceeded smoothly for the most part despite many unstable factors. However, its fatal flaw of "Gestalts relapsing", combined with a product of coincidence - "Replicants attaining self-awareness" ultimately decided its failure. Since the original Gestalt, Gestalt Nier died, all "Gestalts" will eventually relapse and die out. As a result, since the "Replicants" possess no reproductive abilities and need to copy the Gestalt's data to regenerate their bodies, they will die out within this generation. It is not known whether Replicant Nier, who slew Gestalt Nier, put the destruction of the world at his own hands and Yonah on a balance and intentionally chose the latter or not. However, as long as a new original Gestalt remains to be found, humanity will certainly become extinct in the near future.”

I remember reading this back in 2010 and thinking “maybe the next game will be about the next original gestalt” but we got Automata lol