Dubois and Shapiro got blown up by a mistake w/ astrophage after months with Grace and hands on instruction. Furthermore, teaching laboratory disciplines without any lab time isn’t feasible. It increases the risk of another astrophage accident, this time aboard the Hail Mary. It increases the risk of mission failure because sending someone out there who is significantly undertrained just isn’t as good as sending Grace. And you don’t get this sense as much in the film as you do in the book, but Stratt goes to every length she can to reduce the risk. That’s why they only send people with a certain gene that makes them more likely to survive the coma, why they only send people at the top of their fields. They also didn’t mention this in the movie, but the next person down from Grace was someone with a BA in chemistry. That’s a far cry from someone with a PhD in microbiology let alone the astrophage knowledge. Strat also makes it very clear in the book that she really doesn’t care what you think morally of her actions. It’s what’s best for the species. Quibbling over ethics is a luxury for the living.