Motivation for key plot element in Blade Runner 2049

_{Note: this answer contains spoilers for both Blade Runner and Blade Runner 2049.}

I don't know if you are aware of the multiple cuts that were released of the original movie. The theatrical cut ends with the couple (Deckard+Rachael) running away, but it lacks some details that Ridley Scott had planned. Later on, he released the "Director's Cut" that changed everything.

If you consider only the theatrical cut, then the child of both is a hybrid, but the director's cut reveals that Deckard is, in fact, a replicant. There is a scene where he dreams about a unicorn. The last scene of the movie shows him noticing an origami unicorn on the floor, left there by another cop. Dreams are one of the keys to determine whether someone is a replicant. As they are implanted, real people know the dreams the replicants have. I think there are some issues with the original plot to determine that Deckard is a replicant, but at the end of the day, I can't go against Ridley Scott himself, who confirmed this theory.

Moving to Blade Runner 2049, now you know the child is born from two replicants. So Deckard, just as Rachel, is a special creation of Tyrell (he even gets old!). Denis Villeneuve avoids addressing this with certainty, so that Wallace could try to recreate Rachael and fail. But he hints that Wallace suspects about Deckard being a replicant when he captures Harrison Ford's character.

So Rachael is not the key, at least not alone. Both are the key, or they form a combination of key and lock.

The child, then, is the combination of both, the fruit of life directly reproduced by replicants.

They have not yielded the necessary information to replicate the issue, by virtue of the fact they are still looking for the child.

Logically, you are correct; they have Rachel's DNA. The fact the records were destroyed was an overt attempt to conceal as much as possible from Wallace, so it's possible that her DNA holds some key. However, it's clearly not enough. Clearly the 'new rachel' cannot conceive hybrid children; otherwise, the search would be 'over'. They need a complete set of records, including Deckard.

Rachel is a replicant, Deckard was a human. The child is a hybrid. By all accounts, this was considered impossible. The answer to how this was infact possible can only be found by an examination of the subject itself.

From multiple viewings of BR2049, I noticed Wallace says to Deckard that, "I have the lock and the key, yet the key doesn't fit; the door remains locked. I need the child." This is when he is holding the skull of Rachel in front of Deckard, implying that, though he's tried with what he has to discover the secret to reproduction, he can't figure it out from bones. He needs the hybrid / Replicant child to fully understand, because from the child he can examine it, dissect it, to understand fully the child and how it was created.

While Wallace doesn't explain why he couldn't understand Rachel's DNA for the reproduction, it can be inferred that the blackout that wiped out the data on the Nexus 7s is needed to fully understand it, or a live specimen that can be explored.

Wallace had a perfect copy of Rachel, but that still wasn't enough. To be clear, he had a perfect copy of her, and Deckard was just lying about the eye color. You can easily fact check this by rewatching the scene in BR1 where Rachel is taking the Voigt-Kampff test that zooms in on her eye for a long period: her eyes are brown in BR1 and her clone's eyes are brown in BR2.

So Wallace has the lock (the mother), but not the key (the father). He has strong reason to believe Deckard is the key, but if he's not going to cooperate or if he also doesn't have the answer Wallace is looking for (whether that be in his DNA, or him giving up the location of his daughter), the child is the only remaining lead.

Having recently rewatched, particularly the scene in which Wallace 'assesses' the new model replicant before gutting her womb: I've re-evaluated the question and my previous answer.

9 Planets... a Child could count to 9 on it's hands. By now we should own the stars.

Wallace brought the replicants under control, and the replicants (more specifically the mistrust of them creating the societal trend to 'lose their appetite for slaves') has become a limit on the growth of Wallace' Empire.

Wallace's replicants are at the forefront of other planets' colonisation, the trailblazers for humanity's expansion into the cosmos. Regardless of whether these agents of humanity are finding new conditions for life, or simply strip-mining planets for resources, they are at the tip of the spear.

Wallace is a capitalist overlord with multigenerational ambitions: he is interested in building a legacy, beyond wealth. The only way to colonise the galaxy, in his name and in his image (as God), is to create a replicant species capable of pro-creation: a new race under his design, aboard ships full of replicants who will re-produce (as is necessary for 200+ year journeys).

We don't know if it's possible to re-animate replicants after stasis of 200+ years (or whatever the journey may be); that's irrelevant to Wallace. He see's himself as God, and wants to make sure the species he creates continues long after his death. The only way to achieve this is procreating replicants; and Rachel's 'Miracle' is the key to this, hence her importance.