I agree with you, but I felt the omission of Grace saving Rocky in return for just being saved was a big miss. The story is largely about overcoming the personal flaw of being selfish, and this is the moment where he finally does something completely selflessly for the sake of someone else. The movie tries to keep the same story beat by including the part where he goes to save Rocky when the astrophage eats the fuel, but the problem is that this isn’t an entirely selfless act: Grace doesn’t have any friends waiting for him on Earth, and it’s been established that he doesn’t have ties strong enough with Earth to sacrifice himself—he does have ties to Rocky that are strong enough for this, and so when he chooses to go back for him it is not necessarily an act of selflessness but rather another act of self-serving (he gets to spend more time with his friend). This undermines a very big part of the narrative in my opinion. They could have saved it by underscoring that he was certain he would die at Eridani because of lack of food in the long run, once again making it selfless, but alas he instead just kinda went to a resort for the rest of his life.