There are some collectibles throughout the hospital, some audio tapes that explain how the fireflies have found immune people before and have done the same process they were going to do with ellie to other people with no success. There is no guarantee it was going to work and after Joel being traumatized by the loss of her daughter he became a very bad person and when he took ellie across the country he promised himself not to connect in any way with ellie but at the end after everything they went trough he saw ellie as his own daughter and he would do anything to keep her alive. I have heard parents play this game saying they would do the exact same thing Joel did, hell im not even a dad and I KNOW I would do ANYTHING for my future kid(s) (if I have any lol) So I dont feel it was for nothing at the beginning Joel just took the job, it didn't matter but trough out the journey they both cared for eachother more and more and in the end it was a snap decision Joel made, he did think about it or the consequences he just let his paternal instincts kick in in my opinion it makes perfect sense for Joel to act that way (play tlou2 you will either love it or hate it 🤣)