To put it simply, and outside of any story implications, the film wouldn't have got greenlit without Ford's involvement. There had been a few attempts over the years to get a sequel off the ground but without Ford's involvement they didn't get anywhere. Deckard wasn't a genetic engineer, so I doubt he would know the details of Rachel's 'specialness' It's implied that Tyrell showed him her incept file after the VK test in the original film which was how he knew about the implanted memory of the spider, but I doubt it mentioned he had a functioning reproductive system otherwise he wouldn't have been surprised when she became pregnant.
The replicant resistance didn't need to question Deckard - they already knew everything he knew, They had contact with him before the baby was born and Rachel died, and together theyworked out the details of the plan to hide the baby. He says that showed them how to scramble the DNA records, and after the birth his part in the plan was to disappear. As long as Deckard is alive, there is the possibility that he could be tortured to reveal the names of the replicant resistance leaders who took part in the plan to hide the baby, which then would lead Wallace to Ana. Having him dead removes that possibility.
If they have Ana, the replicants don't need Deckard to help them begin to reproduce - Ana's DNA is all they need, and she would become the 'Eve' of a new subspecies of replicant humans with the ability to reproduce.
On a more scientific level, although a child's nuclear DNA (the sequence of chromosomes that defines the characteristics of the organism) consists of half of the mother and father's nuclear DNA, the mitochondrial DNA (the sequence that controls the chemical conversion processes inside each cell) is only inherited from the mother. It may be that Tyrell's reproductive secret was in Rachel's mitochondrial DNA and they need the child to get it. It's also very difficult to extract complete mitochondrial DNA sequences from bones, hair or teeth as it degrades post-mortem a lot quicker than nuclear DNA (it only exists in the mitochondria of cells in living tissue). This may also be why they had removed the flesh from Rachel's bones (which is mentioned by Wood Harris' character during the path lab scene) before burying them.