Why was the final stage of Project Gestalt postponed for hundreds of years?

Per information in both Grimoire NieR and the NieR Automata World Guide, the original Nier, aka Shadowlord, entered an agreement with the Hamelin Organization to save Yonah by putting her in cryogenic stasis for 1000 years while he provided purified maso and they would wait it out until the WCS-causing maso was gone from the world. This deal was made no earlier than 2053

By the time Replicant begins, it's 3465, well beyond the 1000 year deal Nier made with Hamelin. Even if we go by Gestalt's timeline, he made this deal no earlier than 2049, and the game begins in 3361, also more than 1000 years. During this time, the same Devola/Popola pair who befriend the replicant Nier support Shadowlord and presumably restrain him to prevent him from going *too* off the rails, but in any case, it was their job to monitor him

Is there any material stating *why* there was such a delay? I can understand why Shadowlord lost his patience and went to claim Yonah's body before Hamelin wanted him to; they had reneged on the original deal, which was already an insanely steep ask, but why the delay?

My only guess is that replicants achieving sentience happened roughly around the year 3000. That might have been enough of a spanner in the works for Hamelin/the Devola and Popola in charge to say "Let's wait this one out a century or so and hope it's a fluke", and a couple hundred years after that, there was a brief Legion resurgence in Jerusalem, but that wasn't due to WCS - that was a replicant putting another replicant's dead body into an Egg of Destruction, which no one could've foreseen